- © 2000 by the Seismological Society of America
The two large earthquakes to occur in México in 1999 were both normal-faulting, intraplate events in the subducted Cocos Plate. There is, however, a significant difference in their locations with respect to the coast, and, hence, with respect to the strongly coupled plate interface. The 15 June 1999 Tehuacán earthquake (H ∼60 km; Mw = 7.0) was located about 200 km inland from the coast (Singh et al., 1999) and in this sense was a typical intraplate event in the subducted slab. The Oaxaca earthquake of 30 September 1999 (H ∼45 km; Mw = 7.5), on the other hand, occurred almost below the coast. As we point out later, the 1999 Oaxaca earthquake is not unique; two other, recent intraplate events had similar characteristics. In some aspects the 1999 earthquake is similar to the great intraplate Oaxaca earthquake of 15 January 1931 (M = 7.8), which caused great destruction in the state of Oaxaca, especially in the city of Oaxaca.
For the 1999 Oaxaca earthquake the National Seismological Service (SSN) of México reported a magnitude of 7.4 and assigned an epicentral location near the coastal town of Puerto Escondido. Since the real-time network coverage of SSN is poor, the depth of the earthquake was poorly determined. A coastal epicenter suggested a thrust earthquake on the plate interface. It was not until the Harvard CMT solution became available that it was realized that it was a normal-faulting intraplate earthquake.
The earthquake caused serious damage in the state of Oaxaca. The heavy rains which preceded and followed the earthquake compounded the loss and presented enormous difficulties in providing emergency help. In this report, we present the plate tectonic framework of the region, the seismological aspects of the earthquake, an isoacceleration map, the attenuation of peak ground motion with distance, a …