Finite Fault Model

Preliminary Result of the Apr 11, 2012 Mw 8.6 Earthquake Off the West Coast of Northern Sumatra

Gavin Hayes, NEIC


DATA Process and Inversion

We used GSN broadband waveforms downloaded from the NEIC waveform server. We analyzed 38 teleseismic broadband P waveforms, 13 broadband SH waveforms, and 56 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 USGS hypocenter (Lon.=2.35 deg.; Lat.=93.07 deg.). The fault planes are defined using the rapid W-Phase moment tensor solution of the NEIC.


Result

The compactness of the source, complexity of waveforms and distribution of aftershocks makes resolution of the main fault plane difficult for this earthquake. Multiple sources may be involved. For a single source model, because of the location of the large (Mw8.2) aftershock 180 km to the southwest of the mainshock, we favor the NE-SW nodal plane (strike=199 deg., dip=80 deg.). The seismic moment release based upon this plane is 1.23e+29 dyne.cm using a 1D crustal model interpolated from CRUST2.0 (Bassin et al., 2000). The second plane is shown also, for comparison (strike=108 deg., dip=87 deg.). Which ever plane is chosen, slip is very high (50-60+ m), the source very compact (high slips 150-250 km in length), and rupture velocity relatively slow (0.5-2.0 km/s, averging perhaps 1.5 km/s).

Also see the Shao & Ji model for this earthquake, with similar results (though waveform fits and alignments are improved, issues needing further work here).

NE-SW Plane

Cross-section of slip distribution



Figure 1. Cross-section of slip distribution. The strike direction of the 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 black 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 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-meters. The number above the beginning of each trace is the source azimuth and below is the epicentral distance. Shading describes relative weighting of the waveforms.




Figure 3.2. Comparison of teleseismic body waves. The data are 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-meters. The number above the beginning of each trace is the source azimuth and below is the epicentral distance. Shading describes relative weighting of the waveforms.




Figure 4.1. Comparison of long period surface waves. The data are 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. Shading describes relative weighting of the waveforms.




Figure 4.2. Comparison of long period surface waves. The data are 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. Shading describes relative weighting of the waveforms.




Figure 4.3. Comparison of long period surface waves. The data are 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. Shading describes relative weighting of the waveforms.




Figure 5. Surface projection of the slip distribution superimposed on GEBCO bathymetry. Red lines indicate major plate boundaries [Bird, 2003]. Gray circles, if present, are aftershock locations, sized by magnitude.


Slip Distribution


NW-SE Plane

Cross-section of slip distribution



Figure 1. Cross-section of slip distribution. The strike direction of the 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 black 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 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-meters. The number above the beginning of each trace is the source azimuth and below is the epicentral distance. Shading describes relative weighting of the waveforms.




Figure 3.2. Comparison of teleseismic body waves. The data are 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-meters. The number above the beginning of each trace is the source azimuth and below is the epicentral distance. Shading describes relative weighting of the waveforms.




Figure 4.1. Comparison of long period surface waves. The data are 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. Shading describes relative weighting of the waveforms.




Figure 4.2. Comparison of long period surface waves. The data are 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. Shading describes relative weighting of the waveforms.




Figure 4.3. Comparison of long period surface waves. The data are 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. Shading describes relative weighting of the waveforms.




Figure 5. Surface projection of the slip distribution superimposed on GEBCO bathymetry. Red lines indicate major plate boundaries [Bird, 2003]. Gray circles, if present, are aftershock locations, sized by magnitude.


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 United States Geological Survey. This web page is built and maintained by Dr. G. Hayes at the NEIC.