Finite Fault Model

Preliminary Result of the Mar 11, 2011 Mw 8.9 Earthquake Offshore Honshu, Japan

Gavin Hayes, USGS


Gavin's Comments:

This preliminary solution allows a broad range of (variable) rupture velocities (0.5-3.5 km/s), in an attempt to address the slow start to the earthquake (peak moment release at 50-100s after OT). Allowing low rupture velocities means slip can occur close to the hypocenter, if preferred by the waveform inversion, as opposed to being forced further away from the hypocenter by the rupture moveout over the first 50s. Currently, we prefer this solution because it places slip in the area between the early aftershocks. However, it is possible that more slip may have occurred to the south, as indicated by the distant aftershocks.


DATA Process and Inversion

We used GSN broadband waveforms downloaded from the NEIC waveform server. We analyzed 40 teleseismic broadband P waveforms, 20 broadband SH waveforms, and 60 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.=142.36 deg.; Lat.=38.32 deg.). The fault planes are defined using the rapid W-Phase moment tensor solution of the NEIC, adjusted to match local slab geometry.


Result

After analyzing waveform fits based on the nodal planes of the rapid WCMT moment tensor, and those more closely matching the slab geometry, we find that a nodal plane striking 195 deg., and dipping 14 deg., fits the data better. The seismic moment release based upon this plane is 4.04e+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 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. Yellow circles are foreshocks over the preceeding 2 days, including the March 9th M 7.2 earthquake.


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.