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Observation_seismology

VIEWS: 18 PAGES: 50

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									                Introduction to
       Observational Seismology
D. Di Giacomo (domenico@isc.ac.uk),
 A. Strollo (strollo@gfz-potsdam.de)
                  Outline

 Seismic Waves


 Earth interior and Global Earth models


 Seismograms     at   different   distance
ranges
Introduction



           Tectonic earthquakes are
           caused by slippage along
               fractures (faults).
              As slip along the faults
            occurs, energy is released
            which travels in the Earth
          interior in the form of seismic
                      waves…
Harmonic wave parameters and their mutual relationship
  NMSOP, Ch. 2                   Body waves
For homogeneous media and small
 deformations the equation of the
   motion for seismic waves is:




Volume change          Change of
 (compression        shape without
and dilatation)     volume change

The equation of the motion has
  two independent solutions:
                                              For a Poisson solid λ = μ,
                                                   and Vp/Vs = √3.
                            P waves
                                                  The P-waves travel
                                               faster and arrive earlier
                         S waves                 in the seismic record
                                                     than S-waves
                    Body waves – P waves
P waves, also known as primary or compressional waves are the first
                arrivals, but generally not destructive.




Particle motion consists of alternating compression and dilation parallel to
 the direction of propagation (longitudinal polarisation). The material
            returns to its original shape after the wave passes.
                    Body waves – S waves
S waves, also known as secondary or shear waves arrive latter, but can
                     potentially be destructive.




Particle motion consists of alternating transverse motion perpendicular to the
                           direction of propagation.
Inelastic attenuation of the seismic waves

  Waves travelling in the Earth medium are affected by a
   loss of energy due to the anelastic properties of the
                       medium itself.

The amount of energy loss per oscillation is defined by the
                       Q factor:
  Q = 2π E/ΔE, with ΔE being the dissipated energy per
                         cycle.
 Low values of Q mean higher attenuation, and vice versa.
  Q is different for P and S waves, with Qα systematically
                        larger than Qb.

  Moreover, the Q factor depends on the frequency: it is
  nearly constant between 0.001 and 1 Hz, but for higher
    frequencies depends on frequency and, in general,
                increases with frequency.
  NMSOP, Ch. 2             Surface waves

According to theory, in the presence of a free surface and/or of a layered half-
                      space, other solutions are possible.

  In the real Earth, due to the presence of the Earth free surface and of the
   layered Earth’s crust, the body waves radiated by shallow earthquakes
                       generate also the surface waves:


Love waves (LQ), V(LQ) ≈ Vs in the near-     Rayleigh waves (LR), V(LR) ≈ 0.92 Vs
            surface layers
                        Rayleigh waves
         Rayleigh waves are a combination of P and SV waves.




Particle motion consists of elliptical motions (usually retrograde elliptical)
     in the vertical plane and parallel to the direction of propagation.
                      Amplitude decreases with depth.
          Material returns to its original shape after wave passes.
                        Love waves
    Love waves result from SH waves trapped near the surface
                     within the upper layers.




Particle motion consists of alternating transverse motions, horizontal
          and perpendicular to the direction of propagation.
                   Amplitude decreases with depth.
       Material returns to its original shape after wave passes.
                            Surface waves
 Surface waves decay more slowly with distance from the epicentre, r-1,
 compared to body waves that decay as r-2. Hence, are more prominent
                     on distant seismograms.

                                      RAYLEIGH
                                                     Example of a Mw 7.7
                                                      event in Vanuatu
                                                      12,250 km away.
  RADIAL COMP.
                                                      Note the Love wave
                               LOVE                     on the transverse
                                                      component, and the
                                                         Rayleigh on the
  TRANSVERSE COMP.                                     radial and vertical.
                                       RAYLEIGH



 VERTICAL COMP.


(Stein & Wysession, 2003)
                      Surface waves
Surface waves are dispersive, i.e. the propagation velocity depends on
                             frequency.
NMSOP, Ch. 2          Surface waves
        Dispersive waves are characterized by two velocities:
• Group velocity U(T), with which the energy of a wave group travels;
• Phase velocity c(T), with which the wave peaks and troughs travel.
                             c(T) > U(T)
               The Mantle Surface waves
Love and Rayleigh may also circle the Earth several times along great circle
                                  paths.
These global surface waves have periods T > 50 s, are characterized by low
  attenuation and are also used to determine the magnitude of very large
                              earthquakes.




   NMSOP, Ch. 2
         Free oscillations (Normal modes)
The waves reflected back from the
   surface into the Earth and the
 surface waves along great circles
   will interfere destructively and
    constructively. Constructive
  interferences can occur only at
certain resonant frequencies, that
are called the normal modes of the
 Earth (in practice, the Earth rings
              like a bell).
    The normal modes can be
  generated especially by great
earthquakes and are important to
   study the deep Earth interior
 (however, they are generally not
considered in routine observatory
             practice).
                                       NMSOP, Ch. 2
Broadband Seismology
                                 Seismic record

 The waveform that we record at
the seismic station is the result of:




                                             Therefore, the signal recorded at
                                             the seismic station has been also
                                                 “distorted” by the transfer
                                              function of the seismometer…
                             Seismic record




Records of a deep earthquake at one station in Germany. The different traces have been
 obtained by filtering the original record with the response curves of some traditional
                      seismometers shown on the left of each trace.
Transfer function of some modern seismometer
                                                Broadband
                                               seismometer
                                            Guralp 60T: natural
                                              period of 60 s.




                           Broadband
                          seismometer
                           Streckeisen
                         STS-2: natural
                         period of 120 s.



                     (In the DS 5.1 of the NMSOP are described
                        the characteristics of some widely used
                                   seismic sensors)
Broadband Seismology




            No unique seismic sensor can cover
           the whole frequency band required to
                 cover the Earth signals…
                         Seismic sensors
Frequency range of Earth signals and
       typical seismometers.


                                        Broadband
                                       seismometer
                                        Streckeisen
                                          STS-2:
                                       natural period
                                         of 120 s.




                                                        Force balance accelerometer
Clinton, 2004.                                                Episensor ES-T.
                   Seismology and Earth interior

Frequency f (Hz)
                                                The seismic
                                                    waves
                                              travelling in the
     0.001 Hz                                  Earth volume
                                                   from the
       0.01 Hz                                seismic source
                                                     to the
        0.1 Hz                                receivers bring
                                                 information
          1 Hz                                    about the
                                                properties of
         10 Hz
                                                  the Earth
                                                  structure.
        100 Hz
Seismic wave phases

              Each arrival or phase is the
              result of a given seismic ray
            that has followed a certain path
                   through the Earth.
             When combined they provide
             multiple pieces of information
            about the physical properties of
               the Earth, in particular the
              seismic velocity distribution.

              Left: An example of some of
                        the wave
               paths followed by various
                         phases
             and their associated form on a
                      seismogram.

              (Stein & Wysession, 2003).
        Global Earth Models: AK135Q and PREM
 Global Earth
models can be
 retrieved by
   inverting
 phase travel
 data (as well
   as other
  parametric
     data).
    Comparison
   between the
 reference Earth
models AK135Q
    (solid lines,
  Kennett et al.,
1995; Montagner
   and Kennett,
1996) and PREM
  (dashed lines,
 Dziewonski and
Anderson, 1981).
Important discontinuities of the Earth: Crust

• Conrad discontinuity: Seismic boundary between the upper and middle
crust that is usually defined by an increase in seismic velocity from 6.2-
6.4 km/sec to about 6.6-6.8 km/sec. The term has fallen into disuse in
recent years due to the lack of universality of such a discontinuity.

• Mohorovicic discontinuity (Moho): the seismic boundary between the
crust and mantle named after A. Mohorovicic who discovered it from the
travel time data in Europe (Mohorovicic, 1909). The velocity contrast
across the boundary is such that the lower crust typically has a
compressional-wave velocity of 6.5-7.4 km/sec, while the uppermost
mantle a velocity greater than 7.6 km/sec with an average value of 8.1
km/sec. The boundary is between 20 and 60 km deep beneath the
continents and between 5 and 10 km deep beneath the ocean floor.
Important discontinuities of the Earth: Mantle

• Transition         zone:
delimited between the
410 km discontinuity
(which represents the
boundary between UM
and TZ) and the 660 km
discontinuity       (which
represents the boundary
between TZ and LM).
The     410     and    660
discontinuities        are
generated by solid state
phase transitions that
produce strong velocity
gradients     and    sharp
seismic discontinuities.
  Important discontinuities of the Earth: Core
• Gutenberg discontinuity (CMB): it marks the discontinuity between mantle and
core at which the P waves drops from ~13.7 to ~8.0 km/s and that of S waves
from ~7.3 to 0 km/s. Indeed, the CMB reflects the change from the solid mantle
to the fluid outer core.

• Inner core boundary (ICB): discontinuity between the fluid OC and the solid IC
Refraction, reflection, and conversion of a wave of a boundary
 In the example below is shown a P wave that hits a boundary separating
  two layers having different seismic velocities. Part of the energy of the
  incoming P wave is transmitted, part is reflected, part is converted and
transmitted as SV wave, and part is converted and reflected as SV wave.
Phase nomenclature: general rules
                Body wave phase nomenclature
   Because of refraction, reflection and conversion, the majority of phases
   have a complex path history between the source and receiver, and are
   described by combining letters representing each portion of a ray path.

K       P wave through the outer core
I       P wave through the inner core
J       S wave through the inner core
PP      P wave reflected at the surface
PPP     P wave reflected at the surface
   twice
SP      S wave reflected at the surface
   as a P wave
SS      S wave reflected at the surface
   as a S wave
pP      P wave upward from the focus,
   and reflected at the surface
sP      S wave upwards from the
   surface, and converted at the
   surface to a P wave.
c       Reflection at the core-mantle
   boundary (e.g. ScS)
i       Wave reflected at the inner
   core-outer core boundary
   (e.g. PKiKP)
                    Earthquake distance
The distance to an earthquake is generally divided between:

Local:            D < 100 km, seismic recordings are strongly
                  affected by shallow crustal structures.

Regional:         100 < D < 1400 km (1° < D < 13°), recordings
                  dominated by seismic energy refracted along, or
                  reflected several times from the crust-mantle boundary.

Upper mantle:         paths 13° < D < 30°, dominated by seismic energy that
                  turns in the depth range 70 to 700 km, a complex
                  part of the Earth's internal structure.

Teleseismic:      D > 30°, direct P-and S-waves relatively simple, but
                  with complex arrivals from traversing the mantle,
                  as well as core and surface reflections.

(Lay & Wallace, 1995; Stein & Wysession, 2003)
Broadband Seismology
             Crustal phases
NMSOP, IS 2.1, p. 7
Moho: Crust-Mantle Boundary




      Pg




                               Here Pn
                              arrives at
           Pn                  station 3
                              before Pg
Local - Regional Distance Range




                      The distance at which the
                       Pn phase arrives at the
                      same time of the direct Pg
                         is called cross-over
                               distance.
Local - Regional Distance Range




                              Records of
                              an event in
                               Germany
                               (NMSOP,
                               DS 11.1)
       Local - Regional Distance Range

   In the real Earth, the crust is
          characterized by
  heterogeneities and the crust
 layering is generally not flat. In
  the example on the right, the
recordings of a regional event in
 Switzerland (NMSOP, Ch. 2, p.
 46) are shown. Here the y-axis
        is the reduced time.
 The time phase arrivals here
 are more complex due to the
dipping layers that characterize
           the crust.


                                        Records of a regional event in
                                      Switzerland (NMSOP, Ch. 2, p. 46)
Local - Regional Distance Range (3)




 NMSOP, Ch. 11, p. 66
Local - Regional Distance Range (4)

                                  Lg is a complex
                               mixture of multiple S-
                               waves reverberations
                               between the surface
                               and the Moho of SV
                                to P and/or P to SV
                               conversions, as well
                                    as of energy
                                scattered at lateral
                                  heterogeneities.
                               Lg follows close after
                               Sg at D < 300 km, but
                                 is well separated
                                      beyond.




         NMSOP, Ch. 2, p. 18
             Broadband Seismology




NMSOP, Ch. 11, p. 3
            Mantle phases
NMSOP, IS 2.1, p. 8
            Mantle phases
NMSOP, IS 2.1, p. 8          PP phase (red), PPP
                            (green), and PcP (blue)
                Core phases
NMSOP, IS 2.1, p. 9           PKP phase (green),
                               and PKPdf (red)
Teleseismic Distance Range




                        NMSOP, Ch. 2, p.
                             24
Teleseismic Distance Range
Travel Times
       Combining these arrivals gives what
         are termed travel time curves.
          The dots represent the arrival
            times of a given phase.
           A travel time curve therefore
          shows results for each phase.
      (Global Earth models can be retrieved by
            inverting phase travel data)
Travel Times




                 S   PP


           PcP

                 P
Travel Times
                             Polarization
     In the real Earth, the particle motion can be in any direction, and
the shape and orientation in space of the ground-motion particle trajectory
 is called POLARIZATION. It differs for different types of seismic waves
such as P, S and surface waves and may be ± linear or elliptical, prograde
or retrograde. It is also influenced by heterogeneities and anisotropy of the
   medium in which the seismic waves propagate and depends on their
frequency or wavelength, respectively. The polarization of ground motion
     may be reconstructed by analyzing three-component seismic
                                recordings.




  NMSOP, Ch. 2

								
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