Finite Fault Model

Updated Result of the Apr 6, 2010 Northern Sumatra Earthquake

Gavin Hayes, USGS


DATA Process and Inversion

We used GSN broadband waveforms downloaded from the NEIC waveform server. We analyzed 45 teleseismic broadband P waveforms, 18 broadband SH waveforms, and 68 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.=2.36 deg.; Lat.=97.13 deg.). The fault planes are defined using the W-phase moment tensor solution of the NEIC.


Result

After comparing the waveform fits based on two planes, we find that the nodal plane (strike=312.7 deg., dip=12.0 deg.) fits the data better. The seismic moment release based upon this plane is 6.848E+027 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.


Moment Rate Function



Figure 2. Source time function, describing the rate of moment release with time after earthquake origin.


Comparison of data and synthetic seismograms



Figure 3.1. Comparison of teleseismic body waves. The data are shown in black and the synthetic seismograms 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-meters. The number above the beginning of each trace is the source azimuth and below is the epicentral distance. Data are shaded according to their relative weighting (transparent waveforms have low weight).




Figure 3.2. Comparison of teleseismic body waves. The data are shown in black and the synthetic seismograms 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-meters. The number above the beginning of each trace is the source azimuth and below is the epicentral distance. Data are shaded according to their relative weighting (transparent waveforms have low weight).




Figure 3.3. Comparison of teleseismic body waves. The data are shown in black and the synthetic seismograms 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-meters. The number above the beginning of each trace is the source azimuth and below is the epicentral distance. Data are shaded according to their relative weighting (transparent waveforms have low weight).




Figure 4.1. Comparison of long period surface waves. The data are shown in black and the synthetic seismograms 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-meters. The number above the beginning of each trace is the source azimuth and below is the epicentral distance. Data are shaded according to their relative weighting (transparent waveforms have low weight).




Figure 4.2. Comparison of long period surface waves. The data are shown in black and the synthetic seismograms 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-meters. The number above the beginning of each trace is the source azimuth and below is the epicentral distance. Data are shaded according to their relative weighting (transparent waveforms have low weight).




Figure 4.3. Comparison of long period surface waves. The data are shown in black and the synthetic seismograms 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-meters. The number above the beginning of each trace is the source azimuth and below is the epicentral distance. Data are shaded according to their relative weighting (transparent waveforms have low weight).




Figure 4.4. Comparison of long period surface waves. The data are shown in black and the synthetic seismograms 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-meters. The number above the beginning of each trace is the source azimuth and below is the epicentral distance. Data are shaded according to their relative weighting (transparent waveforms have low weight).




Figure 5. Surface projection of the slip distribution superimposed on GEBCO 2008. The black line indicates major plate boundaries [Bird, 2003].


Gavin's Comments:

This is an updated solution to our initial results, which can be found here. Results were updated with refined P- and SH- wave picks and weighting.


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.

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 the National Earthquake Information Center (NEIC) of the United States Geological Survey. This web page is built and maintained by Dr. G. Hayes at the NEIC.