A Report on the Atoyac, Mexico, Earthquake of 13 April 2007 (Mw 5.9)

Abstract

An inslab earthquake occurred below Atoyac, Guerrero, Mexico, on 13 April 2007 ( Mw 5.9) at a depth of 37 km. It was strongly felt in Acapulco (Δ ∼65 km) and Mexico City (Δ ∼270 km); the peak ground accelerations (PGAs) at soft sites in these two cities were as high as 200 cm/s2 and 12 cm/s2, respectively. This was the first significant earthquake in Mexico since the recent strengthening of strong-motion and broadband networks in the country. This was also the first moderate event for which ground-motion maps for Mexico City were produced and distributed without human intervention in near real time. In this paper, we report on the source characteristics and the tectonic implications of the earthquake and the ground motions that it produced. We also discuss the performance of the seismic alert system (SAS) and near real-time generation and distribution of expected ground-motion maps for Mexico City during the earthquake. Our analysis of the Atoyac earthquake is based on broadband seismograms and accelerograms from stations operated by the National Seismological Service (SSN) of the Instituto de Geofisica (IG) at the Universidad Nacional Autonoma de Mexico (UNAM), and the Instituto de Ingenieria (II), UNAM. We also used data from an accelerograph operated by the Centro Nacional de Prevencion de Desastres (CENAPRED) in Acapulco. About 35 aftershocks with M ≥ 3.4 were reported by SSN in the first four days of the earthquake, the largest being one of magnitude 5.3. The SSN network is sparse. However, the accelerograph network operated by the II is relatively dense in Guerrero (Anderson et al. 1994) where a mature seismic gap has been identified (Singh et al. 1981). Merged data from both networks permit reliable location of earthquakes in the region. As the accelerograms from the II network are not …