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March/20/2012 (Mw 7.4), OAXACA, Mexico

Shengji Wei, Caltech

Location of Epicenter


slip map of 2011 Tohoku-oki earthquake The Oaxaca earthquake is located near the boundary between the Cocos plate and the North American plates. These plates are converging at about 6 cm/year due to the subduction of the Cocos plate beneath the North American Plate. The upper figure shows the plate motion and the main rupture zone. The earthquake happens on the interface between the two plates. The lower figure shows a depth profile of the slip model, the corresponding moment rate plot is shown in the upper inset.

DATA Process and Inversion

The source model is obtained by inversion of GSN broadband data downloaded from the IRIS DMC. We analyzed 36 teleseismic P waveforms and 10 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). The epicenter location and point source mechanism (USGS WPhase Moment Tensor Solution) are based on the information provided by NEIC (Lon.=-98.188° Lat.=16.662° depth=20km). The fault plane with strike of 283 degree and dip of 13 degree is used here. 1D velocity model is extracted from the CRUST2.0 global tomography model (Bassin et al., 2000).


This is a relatively simple subduction zone earthquake, most of the enery was released in the first 10s and the slips were compactly distributed in an area of ~30km * 30km.

Cross-section of slip distribution

Figure 1: Upper: the source time function showing the time evolution of released moment rate. Lower: cumulative slip (arrows show slip vectors, and color coding shows amplitude) and isochrons of the seismic rupture. The rupture times are given relative to the onset of slip at the epicenter.

Comparison of data and synthetic seismograms

Figure 2: Comparison of the observed (black) and modeled (red) teleseismic seismograms (in displacement). Station names are indicated to the left of the traces along with the azimuths and epicentral distances in degrees. Peak amplitude in micron of data is indicated above the end of each trace.



(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.

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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