earthquake focal mechanisms and waveform modeling
TRANSCRIPT
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Earthquake FocalMechanisms and Waveform
Modeling
Rengin GökLawrence Livermore National Laboratory
USA
RELEMR Workshopİstanbul 2008
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Gudmundar E. Sigvaldason
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The Dynamic Earth, USGS
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Scalar seismic moment Mo = μAuμ is the shear modulus of the rocks involved in the earthquakeA is the area of the rupture along the fault (LxW)u is the average displacement on A.
u
L
w
The size of the event
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Type of faulting and slip directionStrike, Dip, and Rake
Strike, the azimuth of fault plane
Dip, angle with the horizontal
Rake, angle between slip vector and strike
(Shearer, 1999)
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Extracting source parameters from waveforms?
First motion polarity Waveform modeling of
long period body and surface waves
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First motion polarities
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Stein & Wysession, 2003
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SOMETIMES FIRST MOTIONS DON’T CONSTRAIN FOCAL
MECHANISM
Few nearby stations, so arrivals are near center of focal sphere
Mechanism has significant dip-slip components, so planes don’t cross near center of focal sphere
Additional information is obtained by comparing the observed body and surface waves to theoretical
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Vp2 Vs2density2
Vp1Vs1 density1
EpicentralDistance ()
Dep
th (
h)
Source
Receiver
Source + Path + Receiver responseLayer1
Layer2
Path
Seismic signals
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Ground motion recorded on seismogram as a combination offactors
Frequency domain
SYNTHETIC SEISMOGRAM AS CONVOLUTION
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Source signature around the epicenter
Stein & Wysession, 2003
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Stein & Wysession, 2003
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Relation between fault planes and stress axes
P-axis bisects dilatational quadrant
T-axis bisects compressive quadrant
B-axis intersection of both nodal planes
Stein & Wysession 2001
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(P)Pressure – Smallest moment (T)Tension – Largest moment (N)Null The eigenvalues are magnitude and
the eigenvectors are the axes The axes and their eigenvalues are
composed of strike,dip and rake
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Moment Tensor Basics
A moment tensor is a complete description of equivalent forces of a general seismic point source (Jost and Herrmann, 1989) in an elastic medium(Shearer 1999).
M =
(Shearer, 1999)
333231
232221
131211
MMM
MMM
MMM
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Moment Tensors ( 9 components)
Stein & Wysession, 2003
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EXPLOSION
EARTHQUAKES
(DOUBLE COUPLE)
OTHER SOURCES (CLVD)
Dahlen and Tromp, 1998
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Earthquakes are mostlydouble-couple (DC)
ISO DC CLVD
P amplitude S amplitude
Bodywave radiation patterns
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Julian et al., 1998
Surface wave radiation patterns
(showing DC only)
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TELESEISMIC
BODY WAVES
Initial portion of seismogram includes direct P wave and surface reflections pP and sP
Hence result depends crucially on earthquake depth and thus delay times
Powerful for depth determination
Gives information about rupture processes.
Stein & Wysession, 2003
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Regional Waveform Modeling in Eastern Mediterranean Region
The broadband network coverage is increasing
The velocity models are getting improved
Rupture direction and depth effects
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Very complex regional wave propagation pattern
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WENA(Western Eurasia North Africa) ModelThe WENA1.0 model is constructed by joining a regionalized crustal model ofWestern Eurasia and North Africa to a high-resolution sediment model and ana priori upper mantle model (Pasyanos, 2004)
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Sum to produce any arbitrary mechanism
Three fundamental mode synthetics
Combining fundamental mechanismswith earth response (Green’s functions)
SS
DS
DDDreger, 1999
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Vertical
Radial
Transverse
VSS
VDS
VDD
RSS
RDS
RDD
TSS
TDS
TDD
Synthetic Seismogram
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• Offset correction (Removal of mean)
• Rotation for angle of radial direction
• Instrument response removal
• Low-pass filtering
Observed data Synthetic data
Modeling
•Inversion ( e.g. Dreger, 1996)
•Forward modeling (search the best fit for the range of strike,dip and rake) (Walter WR, 1992)
• Green’s functions for unit source
• Combined with three fundamental faults
Regional Waveform Modeling
(Body + Surface Waves)
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.. small event , good fit
Al-Thoubi et al., GSF 2007
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Resources
NEIC fast moment tensors - from teleseismic P waveforms http://gldss7.cr.usgs.gov/neis/FM/fast_moment.html Harvard CMT solutions - Centroid moment-tensor (from teleseismic long period body waves)
http://www.seismology.harvard.edu/projects/CMT/ EMSC rapid source parameter determination - European-
Mediterranean Seismological Centre - uses P & S waves –results in 24 hours
http://www.emsc-csem.org/ NEIC broadband depths and fault-plane solutions http://neic.usgs.gov/neis/nrg/bb_processing.html Swiss (ETHZ) moment tensor solutions –Regional moment
tensor inversion
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GMT 2008 May 21 18:25:34
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ASH
EVID: 0 DATE: 2007 115 04:19:00.099
Depth = 10 km
Mw = 5.07035
Moment = 5.01791509E+16 Nm
Strike = 262
Dip = 59
Rake = 75
trn rad ver
ASH(388.72)
100 200 100 200 100 200
SGS19()
100 200 300 100 200 300 100 200 300
ABKT()
200300400500 200300400500 200300400500
SGS04()
200300400500600700800900200300400500600700800900200300400500600700800900
SGS27()
3004005006007008009001000 3004005006007008009001000 3004005006007008009001000
SHA()
200300400500 200300400500 200300400500