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PROBABILISTIC SEISMIC HAZARD ASSESMENT

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					                                                                                                                                                                   PROBABILISTIC SEISMIC HAZARD ASSESMENT FOR BULGARIA
                                                                                                                                                                       AS A BASIS FOR A NEW NATIONAL BUILDING CODE
                                                                                                                                                                             D.Solakov, S.Simeonova, L. Christoskov, P. Trifonova and I. Alexandrova
                                                                                                                                                                        National Institute of Geophysics, Geodesy and Geography - BAS, Sofia, BULGARIA

             INVESTIGATED TERRITORY AND SCOPE                                                                                                                                    INPUTS TO THE PROBABILISTIC SEISMIC HAZARD ANALYSIS (PSHA)
              The territory of Bulgaria - a typical example of                                                                                                            The most important input required is an earthquake                                                                                                                                                                                                                                                                                                                                        SEISMIC HAZARD ANALYSIS                                                                                                                                                                                                                                                                                   a)        100
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                                                                                                     Seismological data base               Geological and geophysical                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                


                                                                                                                                                                          catalogue for the region of the study. For this purpose a
                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                          (1
                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                               /Т

                                                                                                                                                                                                                                                                                                                                                                                                                                          In the present study a PSHA was performed, using
                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                    ) 1/2

             high seismic risk area in the eastern part of the
                                                                                                                                                   data base
                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                           10




             Balkan Peninsula. Bulgaria contains important                                                                                                                catalogue for the region bordered by 39.50 - 47.50 N and                                                                                                                                                                                                  the so-called deductive method, which deduces what are                                                                                                                                                                                                                                                                                                                                                                                                                                                         
                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                        (1 /
                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                               Т ) 1/2
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                                                                                                                        Seismotectonic model
                                                                                                                                                                          190- 300 E was compiled (25 000 earthquakes). Data are                                                                                                                                                                                                    the causative sources, characteristics, and ground




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                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                 
             industrial areas facing considerable earthquake
             risk. Moreover, the seismicity of the neighboring                                                            Maximum magnitude
                                                                                                                      evaluation for seismotectonic
                                                                                                                                                                          unified and standardized in accordance with requirements                                                                                                                                                                                                  motions for future earthquakes (MCGuire, 1993). More                                                                                                                                                                                                                                                                                                                                                                                                   0.1
                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                         0.1




             countries, like Greece, Turkey, former Yugoslavia and
                                                                                                                            model elements
                                                                                                                                                                          of the international seismological centers. The intensity                                                                                                                                                                                                 specifically, a version of machine code EQRISK
             especially       Vrancea-Romania         intermediate                                                                                                        estimations are based on MSK intensity scale and the                                                                                                                                                                                                      (MCGuire, 1976), is used with PGA as a hazard parameter.                                                                                                                                                                                                                                                                                                                                                                                              0.01
                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                 1                             10
                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                    Години
                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                             100                         1000
                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                        0.01
                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                 1                               10
                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                   Години
                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                             100     1000

                                                                                                                        Seismic sources model
                                                                                                                                                                          magnitude estimations are equivalent to MS magnitude                                                                                                                                                                                                      The version of machine code EQRISK was developed and                                                                                                                                                                                                                                                                                                                                                                                                    10
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             earthquakes influences the seismic hazard in
                                                                                                                                                                          scale. Using a space-time magnitude dependent window the                                                                                                                                                                                                  used in practice for probabilistic hazard assessment in                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                 


             Bulgaria as well.
                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                               (1
                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                              (1 /                                                                                                                    /Т
                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                 1                   Т ) 1/2                                                                                                               ) 1/2

                                                                                                      Magnitude-frequency
                                                                                                           relations
                                                                                                                                           Ground motion attenuation
                                                                                                                                                 relationships            aftershocks were identified and removed. The threshold                                                                                                                                                                                                    Bulgaria. It gives the following advantages: 1) usage of                                                                                                                                                                                                                                                                                                                                                                                                                                                                                           0.1



              Seismic hazard is the probability that various                                                                                                                                                                                                                                                                                                                                                                       different types of attenuation models, including
                                                                                                                                                                          magnitude MS=4.0 was accepted in order to insure space




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                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                
             levels of strong ground motion will be exceeded                                                                                                                                                                                                                                                                                                                                                                        arbitrary functions of M, R and h; 2) allows different
                                                                                                                      Model of uncertainties in
                                                                                                                                                                          homogeneity (over the territory of Bulgaria) of the                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                         0.01



             during a specified time period at a site. The ground                                                         seismic input
                                                                                                                                                                                                                                                                                                                                                                                                                                    types of laws for different sources; 3) depth is
                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                           0.01




                                                                                                                                                                          catalogue data.
             motion levels may be expressed in terms of peak                                                            Earthquake occurrence                                                                                                                                                                                                                                                                                       included as a random uncertainty (each source is                                                                                                                                                                                                                                                                                                                                                                                                      0.001


                                                                                                                                                                             From the analysis of the depth distribution it was
                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                     1                         10            100                         1000        0.001
                                                                                                                               model                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                         1                              10                              100     1000


             ground acceleration (velocity, displacement) and/or
                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                    Години

                                                                                                                                                                                                                                                                                                                                                                                                                                    described with its own depth distribution); 4) source Fig.3 Completeness of the data (a-
                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                             Години




             peak response spectral amplitudes for a range of                                                                                                             recognized that most of earthquakes in the considered                                                                                                                                                                                                     mechanism is included as a random uncertainty;           shallow quakes and b –intermediate)
             frequencies.
                                                                                                                      Seismic hazard assessment
                                                                                                                                                                          region are shallow (the maximum density of seismicity
                                                                                                                                                                          involves the layer between 5 and 25 km). Vrancea area (in                                                                                                                                                                                                5) allows point and circle sources as well as sources between 2 circles with a
              Seismic hazard maps for the New National Building
                                                                                                                  Fig.1 Flow chart for seismic hazard assessment


                                                                                                                                                                          Romania) is the only part of the considered region that is                                                                                                                                                                                         common center; 6) allows non continuous sources and fault sources; 7) allows Monte-
             Code of Bulgaria were developed using the                                                 Fig.1 Flowchart for
                                                                                                    seismic hazard assessment                                             characterized with the occurrence of intermediate                                                                                                                                                                                                  Carlo sensitive analysis; 8) computation of hazard in terms of PGA and spectral
             procedure showed in Fig. 1. The basic approach used                                                                                                                                                                                                                                                                                                                                                             amplitudes could be performed with one run of the program.
                                                                                                                                                                          earthquakes (h>60km up to 200 km).                         Fig. 2. The spatial pattern of seismicity for the
             for the creation of ground motion maps incorporate                                                                                                                                                                                                                                                                                                                                                                    The seismic hazard map for PGA on rock corresponding to the 475 years return
             in GIS mode the source-geometry, earthquake                                                                                                                   Additionally, for intermediate earthquakes in Vrancea       considered region (within a space window
                                                                                                                                                                                                                                            39.5 - 47.5 N and 190- 300 E, M4.0)                                                                                                                                             period is presented in Fig.6a. To limit the damage of buildings and financial loss in case of
             occurrence model, the strength of the earthquake                                                                                                             (Romania), a new Romanian catalogue (ROMPLUS) was also
                                                                                                                                                                                                                                                                                                                                                                                                                             weaker earthquakes with higher frequency of occurrence, a second hazard level in EC8
             sources, and the appropriate attenuation relations.                                                                                                          considered. The size of the quakes is given in terms of
                                                                                                                                                                                                                                                                                                                                                                                                                             is recommended, corresponding to a recurrence period of 95 years (Fig. 6b). Both hazard
        1
                                                                                                                                                                          moment magnitude Mw.                                                                                                                                                                                                                               maps are generated in ArcGIS 9.2.
b)                                                         Because the compiled catalogue is used for seismic hazard analyses it was
                                                        necessary to examine the completeness of the data using Stepp’s test (1971). The
             
                (
                1/Т 1
                   ) /2
       0.1
                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                           Legend
                                     6.0M<7.0

                                                        test was applied to shallow and intermediate earthquakes separately (Fig.3 a) and                                                                                                                                                                                                                                                                                                                                                                                                                                                        The third required input is the earthquake occurrence model. In the





                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                         Faults
               7.0M


                                                        b). The results show that the compiled catalogue for shallow earthquakes can                                                                                                                                                                                                                                                                                                                                                                                                                                                        present study a truncated exponential distribution is assumed as seismicity
      0.01


                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                            model for each seismic source. The estimated b-values range from 0.6 to 0.8
                                                        be considered complete in the last 400 years for magnitudes larger than 7.0; 250-
                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                           Modeled line                     and from 0.6 to 0.9 for seismic sources in and outside Bulgaria
                                                        300 years for MS≥6.0; 120 years for MS≥5.0 and after 1900 for MS≥4.0 (Fig. 3a).                                                                                                                                                                                                                                                                                                                                                                                                                    source
     0.001
                          100
                            Години
                                                 1000
                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                            respectively. Magnitude of the maximum potential earthquakes (Mmax) for
        For intermediate earthquakes (in Vrancea, Romania) the Stepp's test indicates that the                                                                                                                                                                                                                                                                                                                                                                                                                                                                                              seismic sources in Bulgaria was estimated using functions that relate
 ROMPLUS catalogue can be considered complete in the last 600 years for magnitude larger                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                    magnitude to length of the defined tectonic structures in the region
 thanor equal to 7.0 (Mw≥7.0) and about 200 years for Mw≥6.0 (Fig.3b). The test results imply that it is                                                                                                                                                                                                                                                                                                                                                                                                                                                                                    (relations used in the analysis are presented in Wells and Coppersmith,
 possible to create homogeneous data samples by determining intervals over which earthquakes in                                                                                                                                                                                                                                                                                                                                                                                                                                                                                             (1994). The Mmax estimates are matched to the maximum expected magnitudes
 different magnitude classes are completely reported.                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                       that are proposed by Boncev et al. (1982).
 The second required input is a designation of earthquake sources in the considered region. In                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                 A final input required is a designation of a ground motion attenuation
                                                                                                                                                                                                                                 Fig.4 Seismic source models                                                                                                                                                                                                                                                                                                                                relationship. The attenuation relationship proposed by Ambraseys et al.
the present study seismic sources were identified on the base of comparison of the geological
and geophysical data with the seismological information (historical and instrumental seismicity.)                                                                                                                                                                                                                                                                                                                                                                                                                                                                                           (1996) was chosen for the present study. It is based mostly on European
                                                                                                                                                                                        Thus the compiled seismic source model consists of simple area                                                                                                                                                                                                                                                                                                                                      data and a simplified soil classification is used. It covers large ranges in
The basic characteristics (geometry, geographical extend, earthquake recurrence and maximum
                                                                                                                                                                                        sources and mixed type area sources. The seismic source                                                                                           Legend                                                                                                                                                                                                                                            distance (up to 200 km) and magnitude (4.0≤M≤7.5). The PGA(g) is given by the
earthquake magnitude) and relation to the regional tectonic framework for each seismic source                                                                                                                                                                                                                                                                              Earthquakes

                                                                                                                                                                                        Vrancea was divided to two sub-sources V1 and V2 (based on                                                                        Faults                                                                                                                                                                                                                                                            expression. For shallow earthquakes:
were defined. Two alternative source models were generated (Fig. 4).                                                                                                                                                                                                                                         V2
                                                                                                                                                                                        epicentral density function) with uniform earthquake                                                                                                                                                                                                                                                                                                                                                                                    log10Y=-1.48 + 0.266*M-0.992*log10(R2+3.52)1/2+0.117*Sa+0.124*Ss±σ
                                                                                                                                                                                        frequency and different estimates for maximum potential                                                                                           Seismic sources                                                                                                                                                                                                                                   (1)
                                                                                                                                                                                        earthquake magnitude. Uniform depth distribution in different                                                        V1                                                                                                                                                                                                                                                                                   where R – distance in km; M-earthquake surface wave magnitude, MS; Sa ,
                                                                                                                                 a)                                                                                                                                                                                                                                                                                                                                                                                                                                                         Ss- coefficient for the site geology [rock: (Sa , Ss) = (0,0); stiff: (Sa, Ss) =(1,0);
                                                                                                                                                                                        ranges ([100-150] km for V1 and [80-150] for V2) is assumed for
                                                                                                                                                                                        both Vrancea sub sources (Fig.5).                                                                                                                                                                                                                                                                                                                                                                   soft soil: (Sa, Ss) = (0,1)]; σ denotes the standard deviation.
                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                 For intermediate earthquakes: Attenuation model for PGA presented
                                                                                                                                                                                  2. A slight bimodal distribution is illustrated in Fig. 7c                                                                                                                                                                                                                                                                                                                                                in Lungu et al. (2000) is used in hazard calculations for Vrancea
                                                                                                                                                                                  -The primary mode of the distribution) is for magnitude greater than or                                                                                                                                                                                                                                                                                                                                   intermediate earthquake. The model is given by relation:
                                                                                                                                                                                  equal to 7.5 earthquakes at a distance of 255 to 275 km from the city-
                                                                                                                                                                                                                                                                 Fig.5 Epicenter density function for                                                                                                                                                                                                                                                                                                                           ln(PGA)=3.098+1.053*M*ln(R)-0.0005*R-0.006*H±σ                                                                                                                                                                                                                                                                                                                                                                                     (2)
                                                                                                                                                                                  effect of Vrancea intermediate earthquakes. The secondary mode is a                 Vrancea, intermediate, zone
                                                                                                                                                                                  magnitude 5.0 to 6.0 earthquake at 10 to 20 km from the city- effect of                                                                                                                                                                                                                                                                                                                                          where M-earthquake moment magnitude, Mw, R is hypocentral distance
                                                                                                                                                                                  local seismic sources. The strongest contributor to the hazard (for                                              Деагрегация гр. Габрово 475 г.                                                                                                                                                                                                                                                           and H – earthquake depth.
                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                              Деагрегация гр. Габрово 475 г.
                                                                                                                                                                                                                                                                                                    Деагрегация гр. София 475 г.
                                                                                                                                                                                                                                                                                                   City of Sofia – 475 years                                                                                                                                                                                                                                                                                                           Деагрегация гр. Русе 475
                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                       City of Ruse – 475 г.
                                                                                                                                                                                  PGA) for a recurrence period of 475 years is the Vrancea intermediate            a)                                                                                                                                                                                        0.18
                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                       years
                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                 0.18




                                     Fig.6 Seismic hazard for Bulgaria in PGA: a) for a recurrence period of 475 and b) for                                                       source.
                                                                                                                                                                                                                                                                                                                                                                                                                                                           0.16
                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                  0.3
                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                b)                                                                                                                                                               0.16
                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                 0.35




                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                              Нормиран брой надвишавания
                                                                                                                                                                                                                                                                                                                                                                                                                                                                      Нормиран брой надвишавания
                                                                                                                                                                                                                                                                                                                                                                                                                                                           0.14                                                                                                                                                                                                                                                                                 0.14


                                                          95 years; column represent PGA values in g.                                                                             - The primary mode (well expressed) is for magnitude 5.0 to 7.0                                                                                                                                                                                                        0.12
                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                 0.25
                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                               4.5
                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                0.12                                           0.3
                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                   4.5




                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                          Нормиран брой надвишавания




                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                         Нормиран брой надвишавания
                                                                                                                                                                                  earthquakes at a distance of 10 to 20 km from the city - effect of local
                                                                                                                                                                                                                                                                                                                                                                                                                                                         0.1                                                                                                   5.0                                                                                                                                                                              0.1                                                                                                                5.0



                                                                 DEAGGREGATION
                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                               5.5                                                                                                                                                                                                                             0.25                                                                5.5
                                                                                                                                                                                                                                                                                                                                                                                                                                                         0.08                                                    0.2                                                                                                                                                                                                                            0.08                                                                                                               6.0
                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                               6.0
                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                         Predominant influence of Vrancea
                                                                                                                                                                                  seismic sources. The secondary mode (not well expressed) is for                                                                                                                                                                                                        0.06
                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                               6.5
                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                               7.0
                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                            0.06                                               0.2
                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                   6.5
                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                   7.0


                       De-aggregation of the seismic hazard for a recurrence period of 475 years                                                                                  magnitude greater or equal to 7.5 earthquakes at a distance of more
                                                                                                                                                                                                                                                                                                                                                                                                                                                      0.04


                                                                                                                                                                                                                                                                                                                                                                                                                                                      0.02
                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                 0.15                                          7.5                                                                                                                                                                          0.04


                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                            0.02
                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                           0.15
                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                   7.5




             (probability of exceedance of 10% in 50 years) for PGA was performed for 27 cities                                                                                   than 220 from the city-effect of Vrancea intermediate earthquakes.                                                                                                                                                                                                 0
                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                             0.1
                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                            0                                              0.1




                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                    340
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                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                             320
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                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                       300
                                                                                                                                                                                                                                                                                                                                                                                                                            300




                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                 280
             (administrative centers) on the territory of Bulgaria. The de-aggregation results show




                                                                                                                                                                                                                                                                                                                                                                                                                     280
                                                                                                                                                                                  The strongest contributor to the hazard (for PGA) recurrence period




                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                          260
                                                                                                                                                                                                                                                                                                                                                                                                           260
                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                             0.05                                                                                                                                                                                                                                                                          0.05




                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                    240
                                                                                                                                                                                                                                                                                                                                                                                                   240




                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                            220
                                                                                                                                                                                                                                                                                                                                                                                        220




                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                      200
                                                                                                                                                                                                                                                                                                                                                                                 200




                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                180
                                                                                                                                                                                                                                                                                                                                                                          180
                                                                                                                                                                                                                                                                                                                                                                                                                 )                                                                                                                                                                                                                                                                                        m)




                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                         160
                                                                                                                                                                                                                                                                                                                                                                   160
                                                                                                                                                                                                                                                                                                                                                                                                     (km                                                                                                                                                                                                                                                                                            е (k




                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                  140
                                                                                                                                                                                                                                                                                                                                                            140
                                                                                                                                                                                                                                                                                                                                                                                        ние                                                                                                                                                                                                                                                                                          яни
                                                                                                                                                                                  of 475 years is the near regional seismicity.
                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                           0

             existence of both unimodal and bimodal distribution of earthquake magnitude and distance to




                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                            120
                                                                                                                                                                                                                                                                                                                                                     120
                                                                                                                                                                                                                                                                   7.5                                                                                                                                                                                                                                                                                                    7.5
                                                                                                                                                                                                                                                                                                                                                                               я
                                                                                                                                                                                                                                                                                                                                                                            сто                                                                                                                                                                                                                                                                                                   сто




                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                      100
                                                                                                                                                                                                                                                                                                                                           100
                                                                                                                                                                                                                                                                         7.0                                                                                                                                                                                                                                 0                                                                  7.0
                                                                                                                                                                                                                                                                                                                                                                         Раз                                                                                                                                                                                                                                                                                                   Раз




                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                80
                                                                                                                                                                                                                                                                                                                                     80
                                                                                                                                                                                                                                                                               6.5                                                                                                                                                                                                                                                                                                    6.5




                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                          240
                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                        60
                                                                                                                                                                                                                                                                                                                               60
                                                                                                                                                                                                                                                                                     6.0                                                                                                                                                                                                                                                                                                    6.0




                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                  220
                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                  40
                                                                                                                                                                                                                                                                                                                         40




                                                                                                                                                                                                                                                                                                                                                                                                                                                                      120
                                                                                                                                                                                                                                                                               7.5         5.5                                                                                                                                                                                                                                                                                                    5.5




                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                          200
                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                           20
                                                                                                                                                                                                                                                                                                                   20
                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                        5.0
                                                                                                                                                                                                                                                                                           7.0 5.0 4.5




                                                                                                                                                                                                                                                                                                                                                                                                                                                         100




                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                    0
                                                                                                                                                                                  3. A strong bimodal distribution of earthquake magnitude and distance




                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                              180
                                                                                                                                                                                                                                                                                                         0
                                                                                                                                                                                                                                                                                M                                                                                                                                                                                                                                                                                                      M                      4.5


             ground motion exceedance frequency for PGA. Some of the most representative de-




                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                       160
                                                                                                                                                                                                                                                                                                                                                                                                                                        80
                                                                                                                                                                                                                                                                                                  6.5




                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                            140
                                                                                                                                                                                                                                                                                                                                                                                                                                                                m)




                                                                                                                                                                                                                                                                                                                                                                                                                      60
                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                           )




                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                    120
                                                                                                                                                                                                                                                                                                             6.0                                                                                                                                                                                                                                                                7.5
                                                                                                                                                                                                                                                                                                                                                                                                                                                      е (k                                                                                                                                                                                                                                                                         (km




                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                        100
                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                       7.0
                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                               ие




                                                                                                                                                                                                                                                                                                                                                                                                    40
                                                                                                                                                                                                                                                                                                                                                                                                                           яни
                                                                                                                                                                                                                                                                                                                         5.5
                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                           оян




                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                80
                                                                                                                                                                                  to ground motion exceedance frequency is presented in Fig. 7d.
                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                  6.5
                                                                                                                                                                                                                                                                                                         M                                                                                                              сто                                                                                                                                                                                                                                                                            зст




                                                                                                                                                                                                                                                                                                                                                                                 20




                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                        60
             aggregation plots are illustrated in Figs. 7a-c.
                                                                                                                                                                                                                                                                                                                                    5.0                                                                                                                                                                                                                                                                 6.0
                                                                                                                                                                                                                                                                                                                                                                                                                     Раз                                                                                                                                                                                                                                                                            Ра




                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                            40
                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                               5.5




                                                                                                                                                                                                                                                                                                                                                                  0
                                                                                                                                                                                                                                                                                                                                                4.5




                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                     20
                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                        5.0
                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                    M




                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                            0
                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                4.5

                                                                                                                                                                                  The primary mode of the distribution is for magnitude greater or equal                                                 City of V. TurnovoТърновоyears
                                                                                                                                                                                                                                                                                                           Деагрегация гр. В. – 475 475 г.
                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                      Деагрегация гр. Варна
                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                City of Varna– 475 years 475 г.
             PSHA de-aggregation plots for PGA show the following peculiarities:
                                                                                                                                                                                  to 7.0 earthquakes at a distance 10 to 20 km from the site - effect of         c)                                 Деагрегация гр. Габрово 475 г.
                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                   0.25
                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                            d)                                                                                                                                                                                                                            0.14



             1. Two types of unimodal distribution are identified:                                                                                                                local seismic sources. The secondary mode is a magnitude 7.5 or larger                                                                                                                                                                                                       0.18
                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                         0.12
                                                                                                                                                                                                                                                                                                                                                                                                                                                             0.16




                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                      Нормиран брой надвишавания
                                                                                                                                                                                  earthquakes at a distance of more than 250 km from the city - effect of
                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                  0.2

             - The mode of the distribution is for magnitude 5.0-7.5 earthquakes at a distance of about 5 to




                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                              Нормиран брой надвишавания
                                                                                                                                                                                                                                                                                                                                                                                                                                                                                Нормиран брой надвишавания
                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                         0.1
                                                                                                                                                                                                                                                                                                                                                                                                                                                             0.14




             10 km from the cities – effect of local seismic sources (the example presented in Fig. 7a is the                                                                     Vrancea intermediate earthquakes (the example presented in Fig. 7d is                                                                                                                                                                                                      0.12


                                                                                                                                                                                                                                                                                                                                                                                                                                                             0.1
                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                  0.15                                          4.5
                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                5.0
                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                         0.08




             de-aggregated hazard for the cities of Sofia and Plovdiv). The strongest contributor to the                                                                          the de-aggregated hazard for the city of Varna). The de-aggregation of                                                                                                                                                                                                   0.08
                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                 0.1
                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                5.5
                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                6.0
                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                6.5
                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                     0.06




                                                                                                                                                                                  the hazard (for PGA) for a recurrence period of 475 years shows that
                                                                                                                                                                                                                                                                                                                                                                                                                                                           0.06
                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                7.0                                                                                                                                                                                                                                  0.04


             hazard (for PGA) for a recurrence period of 475 years is the near regional seismicity.                                                                                                                                                                                                                                                                                                                                                        0.04                                                                                                 7.5


                                                                                                                                                                                  the hazard is influenced both from near region seismicity and Vrancea
                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                 0.05
                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                     0.02
                                                                                                                                                                                                                                                                                                                                                                                                                                                         0.02



             - The mode of the distribution is for magnitude greater than or equal to 7.5 earthquakes at a                                                                        intermediate earthquakes
                                                                                                                                                                                                                                                                                                                                                                                                                                                         0
                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                             0
                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                     0




                                                                                                                                                                                                                                                                                                                                                                                                                                               340
                                                                                                                                                                                                                                                                                                                                                                                                                                        320




                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                           340
                                                                                                                                                                                                                                                                                                                                                                                                                             300
                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                         Fig.8 Region with predominant influence of




                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                            320
                                                                                                                                                                                                                                                                                                                                                                                                                      280




                                                                                                                                                                                                                                                                                                                                                                                                                                                                                              320




                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                        300
             distance of more than 200 km from the cities – effect of Vrancea intermediate earthquakes
                                                                                                                                                                                                                                                                                                                                                                                                            260




                                                                                                                                                                                                                                                                                                                                                                                                                                                                    300
                                                                                                                                                                                                                                                                                                                                                                                                     240




                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                280
                                                                                                                                                                                  Examination of maps of hazard distributions enables the investigator



                                                                                                                                                                                                                                                                                                                                                                                                                                                             280
                                                                                                                                                                                                                                                                                                                                                                                             220




                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                      260
                                                                                                                                                                                                                                                                                                                                                                                                                                                     260
                                                                                                                                                                                                                                                                                                                                                                                  200




                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                              240
                                                                                                                                                                                                                                                                                                                                                                                                                                          240
                                                                                                                                                                                                                                                                                                                                                                           180




                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                       220
                                                                                                                                                                                                                                                                                                                                                                                                             )




                                                                                                                                                                                                                                                                                                                                                                                                                                  220
                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                         Vranceaintermediate earthquakes (over 50%,
                                                                                                                                                                                                                                                                                                                                                                    160

                                                                                                                                                                                                                                                                                                                                                                                                         (km




                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                200
                                                                                                                                                                                                                                                                                                                                                                                                                      200
                                                                                                                                                                                                                                                                                                                                                             140




                                                                                                                                                                                                                                                                                                                                                                                             ние




                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                      180
                                                                                                                                                                                                                                                                                                                                                                                                           180
                                                                                                                                                                                                                                                                                                                                                      120




                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                          m)
                                                                                                                                                                                                                                                                    7.5
                                                                                                                                                                                                                                                                                                                                                                                  тоя




                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                              160
                                                                                                                                                                                                                                                                                                                                                                                                                                                     )
             (the example presented in Fig. 7b is the de-aggregated hazard for the for the cities of Ruse and                                                                                                                                                                                                                                                                                      160
                                                                                                                                                                                                                                                                                                                                               100




                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                             е (k
                                                                                                                                                                                  to determine the distance and azimuth to predominant sources, and
                                                                                                                                                                                                                                                                          7.0
                                                                                                                                                                                                                                                                                                                                                                          Раз
                                                                                                                                                                                                                                                                                                                                                                             с                                                           (km




                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                      140
                                                                                                                                                                                                                                                                                                                                                                                       140
                                                                                                                                                                                                                                                                                                                                      80




                                                                                                                                                                                                                                                                                6.5
                                                                                                                                                                                                                                                                                                                                                                                                          ие                                                                                                                                                                                                                                                                                               ни




                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                               120
                                                                                                                                                                                                                                                                                                                                                                            120
                                                                                                                                                                                                                                                                                                                                60




                                                                                                                                                                                                                                                                           7.5                                                                                                                                                                                                                                                                                             7.5
                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                       тоя
                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                         75%, 80%, 85%) on seismic hazard.
                                                                                                                                                                                                                                                                                      6.0
                                                                                                                                                                                                                                                                                                                                                                                                      оян
                                                                                                                                                                                                                                                                                                                                                                    100




                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                    100
                                                                                                                                                                                                                                                                                                                          40




                                                                                                                                                                                                                                                                                 7.0        5.5                                                                                                                                                                                                                                                                                  7.0
                                                                                                                                                                                                                                                                                                                                                                                                  зст                                                                                                                                                                                                                                                                                              азс
                                                                                                                                                                                                                                                                                                                    20




                                                                                                                                                                                                                                                                                                                                                            80




                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                             80
                                                                                                                                                                                                                                                                                           6.5 5.0                                                                                                                                                                                                                                                                                      6.5
                                                                                                                                                                                                                                                                                                                                                                                               Ра                                                                                                                                                                                                                                                                                                 Р
                                                                                                                                                                                                                                                                                                             0




                                                                                                                                                                                                                                                                                                                                                     60




                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                      60
                                                                                                                                                                                                                                                                                 M            6.0 4.5                                                                                                                                                                                                                                                                                             6.0
                                                                                                                                                                                                                                                                                                                                          40




                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                40
             Pleven). The strongest contributor to the hazard (for PGA) for a recurrence period of 475                                                                            their magnitudes. This information can be used to generate scenario
                                                                                                                                                                                                                                                                                                   5.5                                                                                                                                                                                                                                                                                                  5.5
                                                                                                                                                                                                                                                                                                                                20




                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                       20
                                                                                                                                                                                                                                                                                                         5.0                                                                                                                                                                                                                                                                                                  5.0
                                                                                                                                                                                                                                                                                                                          0




                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                              0
                                                                                                                                                                                                                                                                                            M                    4.5                                                                                                                                                                                                                                                                        M                        4.5




             years is the Vrancea intermediate source.                                                                                                                            earthquakes and corresponding time histories for seismic design and
                                                                                                                                                                                  retrofit.                                                                               Fig.7 PSHA deaggregation showing the distribution of earthquake magnitude
                                                                                                                                                                                                                                                                                 and distance to ground motion exceedance frequency for PGA
 The present work is supported by the Ministry of Regional Development and Public Works of Bulgaria

				
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