Location of Aftershocks of the 4 April 2010 Mw 7.2 El Mayor-Cucapah Earthquake of Baja California, Mexico

A magnitude M-w 7.2 earthquake occurred on 4 April 2010 at approximately 50 km southwest of the city of Mexicali, Baja California, Mexico. The regional networks Red Sismica del Noroeste de Mexico (RESNOM) that the Centro de Investigacion Cientifica y de Educacion Superior de Ensenada (CICESE) operates and the Southern California Seismic Network (SCSN) located the main event between the sierras El Mayor and Cucapah at the southeastern end of the Pescadores fault. Twenty-four hours after the origin time of the main event, a temporal seismic network was installed around the faults that ruptured during this event. We used body-wave arrival times recorded by the first 14 stations installed to obtain precise hypocentral locations of this important sequence of earthquakes. Most of the aftershocks located on 6 and 7 April are distributed near the traces of the Pescadores and Cucapah faults, particularly at the southern end of these faults. The spatial distribution of the epicenters indicates that these faults and possibly others may extend southeast of Sierra Cucapah where the faults were buried by sediments and were activated during the seismic sequence. In this area, north of the Sierra El Mayor, where no faults were previously mapped, the epicenters align in the northwest-southeast direction. Most hypocenters have depths less than 15 km, suggesting that an important portion of the seismic slip was shallow. Based on the distribution of focal depths of the aftershocks, we infer that the seismogenic zone must be between 5 and 10 km.