Slip history the 2004 (Mw 5.9) Parkfield Earthquake
(Single-Plane Model)
Chen Ji, Caltech
DATA Process and Inversion
We revise our preliminary
slip model by including the waveforms of 12 1-Hz GPS stations (Contributed
by Kevin Choi, Andria Bilich and Prof. Kristine M. Larson of University
of Colorado at Boulder) and a close strong motion station, Cholame 5W
(CH5W, CISN). 15 GPS vectors distributed by Dr. Nancy King of USGS are
also combined into this inversion. We use a 1D velocity structure modified
from a 3D velocity structure around Parkfield (Thurber et al, 2003).
Green's functions of both static displacement and synthetic seismograms
are calculated using the same structure. We also reduce the size of the
subfault (2 km (along strike) by 1.45 km (downdip)).
Result
The total seismic moment of our preferred single-plane model is 9.4x10^24
dyne.cm, consistent with point source moment tensor solution. The slip
distribution is characterized by two large asperities. One of them is
around the epicenter and has a peak slip of 42 cm, larger than the previous
result. The other is centered 15 km northeast of the epicenter and has
a peak slip about 30 cm. Two asperities are well separated and a large
aftershock (ML=5.0) occurred minutes after the mainshock seems to fill
the gap. The slip distribution also includes a small asparity 30 km northwest
of the epicenter at a depth about 8 km. Because it appears only when
the GPS static vectors are included and its subfaults are associated
with the longest rise time allowed during the inversion, it may not occur
co-seismically. Comparing with the previous result, the near surface
slip are better constrained by the GPS stations right above the fault
plane.
Cross-section of slip distribution
Figure: A big black arrow shows the fault's strike. The color shows the slip
amplitude and arrows indicate the motion direction of the hanging wall relative
to the footwall. Contours show the rupture initiation time and a red star indicated
the hypocenter location. The aftershocks are plotted in black circles whose radius
are normalized by its magnitude (10^(ML/2)). The red circle indicates the hypocenter
of the 1966 Parkfield earthquake.
Comparison of data and synthetic seismograms
Figure: The Data are shown in black and the synthetic seismograms are plotted
in red. Both data and synthetic seismograms are aligned by the P arrivals. The
number at the end of each trace is the peak amplitude of the observation in cm.
Figure: Comparison of static displacements. San Andreas fault system are plotted
in pink and the surface projection of the fault plane is shown by a white box.
A black star indicated the CISN epicenter. It is shifted 1.2 km northeast to
the red star so that the fault plane could match the trace of the San Andreas
fault.
Download (Slip Distribution)
References
Ji, C., D.J. Wald, and D.V. Helmberger, Source description of the 1999 Hector
Mine, California earthquake; Part I: Wavelet domain inversion theory and resolution
analysis, Bull. Seism. Soc. Am., Vol 92, No. 4. pp. 1192-1207, 2002.
Thurber, C., S. Roecker, K. Roberts, M. Gold, L. Powell, and K. Rittger, Earthquake
locations and three dimensional fault zone structure along the creeping section
of the San Andreas fault near Parkfield, CA: Preparing for SAFOD, Geophys.
Res. Lett., Vol, 30, No., 3, 1112., doi:10.1029/2002GL016004, 2003.
GCMT project: http://www.globalcmt.org/
USGS National Earthquake Information Center: http://neic.usgs.gov
Global Seismographic Network (GSN) is a cooperative scientific facility operated jointly by the Incorporated Research Institutions for Seismology (IRIS), the United States Geological Survey (USGS), and the National Science Foundation (NSF).
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