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Finite Fault Model

Preliminary Result of the May 12, 2008 Mw 7.9 Eastern Sichuan, China Earthquake

Chen Ji, UCSB, and Gavin Hayes, NEIC


DATA Process and Inversion

We used the GSN broadband waveforms downloaded from the NEIC data center. We analyzed 17 teleseismic broadband P waveforms, 10 broadband SH waveforms, and 30 long period surface waves selected based upon data quality and azimuthal distribution. Waveforms are first converted to displacement by removing the instrument response and then used to constrain the slip history based on a finite fault inverse algorithm (Ji et al, 2002). We use the hypocenter of the USGS (Lon.=31.10 deg.; Lat.=103.30 deg.). The fault planes are defined using the quick moment tensor solution of the Global CMT.


Result

After comparing the waveform fits based on two planes, we find that the nodal plane (strike=229.00 deg., dip=33.00 deg.) fits the data better. The seismic moment release based upon this plane is 0.115E+29 dyne.cm using a 1D crustal model interpolated from CRUST2.0 (Bassin et al., 2000).

Cross-section of slip distribution



Figure 1. Cross-section of slip distribution. the strike direction of fault plane is indicated by the black arrow and the hypocenter location is denoted by the red star. The slip amplitude are showed in color and motion direction of the hanging wall relative to the footwall is indicated by white arrows. Contours show the rupture initiation time in seconds.


Comparison of data and synthetic seismograms




Figure 2. Comparison of teleseismic body waves. The data is shown in black and the synthetic seismograms are plotted in red. Both data and synthetic seismograms are aligned on the P or SH arrivals. The number at the end of each trace is the peak amplitude of the observation in micro-meter. The number above the beginning of each trace is the source azimuth and below is the epicentral distance.




Figure 3.1. Comparison of long period surface waves. The data is shown in black and the synthetic seismograms are plotted in red. Both data and synthetic seismograms are aligned with the earthquake origin time. The number at the end of each trace is the peak amplitude of the observation in millimeter. The number above the beginning of each trace is the source azimuth and below is the epicentral distance.




Figure 3.2. Comparison of long period surface waves. The data is shown in black and the synthetic seismograms are plotted in red. Both data and synthetic seismograms are aligned with the earthquake origin time. The number at the end of each trace is the peak amplitude of the observation in millimeter. The number above the beginning of each trace is the source azimuth and below is the epicentral distance.




Figure 4. Surface projection of the slip distribution superimposed on ETOPO2. The black line indicates the major plate boundary [Bird, 2003].


CJ's Comments:

This is an update of the previous solution.


Slip Distribution

SUBFAULT FORMAT CMTSOLUTION FORMAT


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.

Bassin, C., Laske, G. and Masters, G., The Current Limits of Resolution for Surface Wave Tomography in North America, EOS Trans AGU, 81, F897, 2000.


Acknowledgement and Contact Information

This work is supported by National Earthquake Information Center (NEIC) of United States Geological Survey. This web page is built and maintained by Dr. C. Ji at UCSB.