Finite Fault Model
Preliminary Result of the Oct 7, 2009 Mw 7.8 Vanuatu Earthquake
Gavin Hayes, NEIC
DATA Process and Inversion
We used the GSN broadband waveforms downloaded from the NEIC data server. We analyzed 16 teleseismic broadband P waveforms, 8 broadband SH waveforms, and 29 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 initial hypocenter of the USGS (Lon.=-13.05 deg.; Lat.=166.18 deg.). The fault planes are defined using the quick 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=324.6900 deg., dip=30.74000 deg.) fits the data better. The seismic moment release based upon this plane is 4.28E+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. 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 4.1. 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 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 4.2. 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 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 5. Surface projection of the slip distribution superimposed on ETOPO2. The black line indicates the major plate boundary [Bird, 2003].
Gavin's Comments:
Poor SH-wave fits (NWAO,TNTI,TATO,PFO,RAR) are downweighted with respect to other body waves. Surface-waves show contamination from second larger earthquake, which do not effect the model due to constraints based on the length and size of the primary rupture. Body-wave only inversions show similar slip distributions.
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.
