Preliminary Result
02/20/2008 (Mw 7.4) , Simeulue Earthquake, Indonesia

A. Sladen, Caltech

DATA Process and Inversion

We used the GSN broadband data downloaded from the IRIS DMC. We analyzed 20 teleseismic P waveforms and 5 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 of the USGS (Lon.=95.978° Lat.=2.778°), and the GCMT solution to determine the dip (7°) and strike (302°) angles. The depth of the hypocenter is estimated to 25 km from the position and depth of the nearby March 2005, Nias earthquake. We require the total seismic moment to be close from the GCMT estimate (1.7e20 N.m). The crust model at the source is taken from Bassin et al. (2000).



Cross-section of slip distribution

Figure: The colors show the slip amplitude and the white arrows indicate the direction of motion of the hanging wall relative to the footwall. Contours are isochrons (in seconds) of the rupture propagation from the the hypocenter location (red star). The rupture velocity is about 3 km/s, and the total seismic moment 1.7e20 N.m (Mw=7.4). The big black arrow shows the fault's strike.

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 micro-meter. The number above the beginning of each trace is the source azimuth and below is the epicentral distance.

Map view of the slip distribution

Figure: Surface projection of the slip distribution. There is little energy released at the hypocenter (red star), but the rupture propagated down-dip to a larger patch with a maximum slip amplitude of more than 1 m. Contours of the slip distribution are every 50 cm. The two yellow stars are the epicenters of the 2004 Sumatra-Andaman and 2005 Nias earthquakes.


Figure: Static-field computed from the solution of the slip inversion. The vertical component of displacement is given by the color-scale while the arrows show the horizontal component.


(Slip Distribution)


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:

USGS National Earthquake Information Center:

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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