Finite Fault Model
Magnitude 9.1 OFF THE WEST COAST OF NORTHERN SUMATRA
Sunday, December 26, 2004 at 00:58:53 UTC
Preliminary Rupture Model
Contributed by Chen Ji, Caltech
DATA Process and Inversion
We used the GSN broadband data downloaded from the IRIS DMC. We analyzed 15 teleseismic P waveforms and 13 SH waveforms selected based upon data quality and azimuthal distribution. Waveforms are first converted to displacement by removing the instrument response and then are used to constrain the slip history based on a finite fault inverse algorithm (Ji et al, 2002). We use the hypocenter from the USGS (Lon.=95.78 deg.; Lat.=3.30 deg.). The fault planes are defined by slightly modifying the quick moment tensor solution from HARVARD (strike=320 deg. and dip =11 deg.)
The seismic moment release on this plane is 3.57x10**29 dyne.cm using a 1D crustal model interpolated from CRUST2.0 (Bassin et al., 2000). The total rupture duration is 200 sec and the peak slip is about 20 m. The rupture propagates northwestward for nearly 400 km with a speed of 2.0 km/sec.
Cross-section of slip distribution
Figure: 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.
Comparison of data and synthetic seismograms
Figure: 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 micrometers. The number above the beginning of each trace is the source azimuth and below it is the epicentral distance.
Figure: Surface projection of the slip distribution. The ocean is plotted in blue and land is plotted in green. The black line indicates the plate boundary (data from Dr. Lisa Gahagan, Paleo-Oceanographic Mapping Project at University of Texas at Austin).
It is noteworthy that the seismic data we used only could constrain the slip
in first 220 sec. Hence, we can not totally rule out later, smaller slip if
it occurred further north.
We notice that the location of the biggest asperity correlates well with a nearly 30 degree bend of the subducted India plate. In the figure shown below, we let the fault plane rotate 7 deg. counterclockwise to match the 0 and 50 km isodepth contours of Gundmundsson and Sambridge (1998). It is clear that the northern boundary of the biggest asperity is along the hinge line of the slab (indicated by arrows). The pink circles are big aftershocks (>5) downloaded from the NEIC. Five of them locate at the north boundary as well. The large slip associating with the bend of the fault plane is a common feature, e.g., 1999 Chi-Chi earthquake (Ji et al., 2003, JGR).
This result also suggests that we need use at least two fault segments with different strikes to model the rupture of this event. We are currently working on such a model and we will provide an update as it becomes available.
(below) Predicted static surface displacements (in meters) for vertical (left) and horizontal (right) components of motion. Based on the single-place finite fault source model shown.
Download (Slip Distribution): SUBFAULT FORMAT
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,
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.
Gundmundsson, O. and M. Sambridge, A regionalized upper mantle (RUM) seismic model, JGR, v. 103, No. B4, 7121-7136, 1998.
This work is supported by Gordon and Betty Moore Foundation Caltech Tectonic Observatory, National Earthquake Information Center (NEIC) of United States Geological Survey, and Caltech's Seismological Laboratory.
Preliminary Earthquake Report
U.S. Geological Survey, National Earthquake Information Center
World Data Center for Seismology, Denver