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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
10
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
1
Seismotectonic model
190- 300 E was compiled (25 000 earthquakes). Data are the causative sources, characteristics, and ground
1
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
1
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
0.1
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, M4.0) 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
6.0M<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
7.0M
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
340
320
320
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
