Abstract

On 21 October 1995, we recorded with a local array an earthquake that occurred at a depth of 165 km in the subduction zone of Chiapas. The Harvard focal mechanism solution indicates a normal fault responding to the down-dip tension of the subducted oceanic crust. This is the first intermediate-depth earthquake well recorded with accelerographs and seismometers in Southeastern Mexico. Peak ground accelerations (PGA) range from 21 to 436 cm/sec2 at hypocentral distances of 174 to 256 km, respectively. The recorded PGAs are larger than those of the Copala, Guerrero, earthquake of 14 September 1995, which was a shallow (16 km) thrust fault with a similar magnitude (Mw = 7.4). The large PGA generated by the Chiapas earthquake are probably due to an enhancement of the signals produced by the up-ward intraslab propagation of energy and are similar to those observed from other intermediate-depth earthquakes in the subduction zone of Japan (Molas and Yamazaki, 1995).

The duration of the strongest shaking increases from about 10 sec in the southeast at the town of San Vicente (close to the Tacaná volcano) to nearly 20 sec in the northwest, in the city of Tuxtla Gutierrez located near the epicenter. Teleseismic P-wave inversion using the Harvard focal mechanism solution indicates that the seismic moment was released in three events with a total duration of about 20 sec. The results of the inversion indicate that the rupture propagated from the northwest to the southeast along a 30-km distance. From spectral analysis, we calculate a total seismic moment release of 5.2 ± 0.5 × 1019 N-m equivalent to an Mw = 7.1 magnitude event. Using three sources with an average depth of 150 km, we were able to reach a reasonable match of the first 40 sec of the displacement records recorded at the broadband seismic stations of Huatulco (HUIG) and Pinotepa Nacional (PNIG). For the station located in Tuxtla Gutierrez (TUXD), we used two sources, since only the first 5 sec were modeled.

The amplitude spectrum at teleseismic distances follows a typical Brune's (1970) θ−2 model. We obtained a corner frequency of 0.045 Hz from the spectra, which is equivalent to a source radius of 15 km and a stress drop of 65 bars assuming a circular fault.

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