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Preliminary Result
09/30/2009 (Mw 7.6), Padang, Indonesia

Anthony Sladen, Caltech

Location of Epicenter

Amount of Slip on Fault

View in Google Earth (requires Google Earth)

Colors show the amount of slip on diferent sections of the fault zone. Two views are shown (either view can be de-selected on the Google Earth sidebar):

  • Circles show a 3-D map of the fault. (The fault map has been raised above the Earth's surface. Click on a circle to get the depth of that fault segment.)

  • Image shows the projection of the fault onto the Earth's surface.

    • DATA Process and Inversion

    We used the GSN broadband data downloaded from the IRIS DMC. We analyzed 23 teleseismic P waveforms and 27 teleseismic SH waveforms 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 epicenter location of the USGS (Lon.=99.961 ° Lat.=-0.789 °). The 1D velocity model is extracted from the CRUST2.0 global tomography model (Bassin et al., 2000). The depth of the epicenter is fixed at 80 km, and the rupture velocity is assumed to be about 2.4 km/sec.

    Result

    The solution is made of one sharp slip patch down-dip of the epicenter. Because of these source characteristics and the unexplained structural origin of this earthquake, we cannot infer which of the two nodal planes corresponds to the rupture plane. Below, we present the solutions (models A and B) obtained for the two fault orientations proposed by the GCMT double-couple solution:
    • MODEL A: strike=193 °, dip=58 °

    • MODEL B: strike=72 °, dip=51 °

    Cross-section of slip distribution



    Figure 1: The big black arrow shows the fault's strike. The colors show the slip amplitude and white arrows indicate the direction of motion of the hanging wall relative to the footwall. Contours show the rupture initiation time and the red star indicates the hypocenter location. Both models have most of the slip down-dip

    Comparison of data and synthetic seismograms





    Figure 2: The Data are shown in black and the synthetic seismograms are plotted in red. Both data and synthetic seismograms are aligned on the P 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 it is the epicentral distance. The fit of the waveforms is equivalent for the two fault models.

    Map view of the slip distribution



    Figure 3: Surface projections of the slip distributions.

    Map view of the surface deformation



    Figure 3: Surface deformation predicted from Models A and B. The vertical component of displacement is given by the color scale, and the horizontal motion by the arrows.


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    (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.

    GCMT project: http://www.globalcmt.org/

    USGS National Earthquake Information Center: http://neic.usgs.gov

    Global Seismographic Network (GSN) is a cooperative scientific facility operated jointly by the Incorporated Research Institutions for Seismology (IRIS), the United States Geological Survey (USGS), and the National Science Foundation (NSF).
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