PRELIMINARY SEISMOTECTONIC SYNTHESIS OF THE 2010 Mw7.2 EL MAYOR-CUCAPAH EARTHQUAKE SEQUENCE, BAJA CALIFORNIA

The Caltech/USGS Southern California Seismic Network and the CICESE Baja California, Mexico Seismic Network have recorded more than 10,000 earthquakes in the El Mayor-Cucapah earthquake sequence. The sequence started with preshocks in March 2010, and a sequence of 15 foreshocks of M>2 (up to M4.4) that occurred during the 24 hours preceding the mainshock. The foreshocks occurred along a north-south trend near the mainshock epicenter. The mainshock that occurred on 4^th of April exhibited complex faulting, possibly starting with a ~M6 normal faulting event, followed ~15 sec later by the main event, which included simultaneous normal and right-lateral strike-slip faulting. The synthesis of the El Mayor sequence reveals transtensional regional tectonics and how Pacific North America plate motion is transferred from the Gulf of California in the south into the San Andreas fault system to the north.

The aftershock zone that has a length of more than 120 km, extends from the south end of the Elsinore fault zone at the US-Mexico border almost to the Gulf of California coast. The aftershocks form two abutting clusters, of about equal length or 60 km each, as well as a 10 km north-south aftershock zone at the epicenter of the mainshock. The event relocations benefit from real-time waveform exchange between the two networks, and the availability of arrival time picks from the web site provided by the CICESE Baja California, Mexico Seismic Network. Even though the Baja data are included, the magnitude of completeness and the hypocentral errors increase gradually with distance to the south of the international border. The spatial distribution of large aftershocks is asymmetric with five M5+ aftershocks located to the south of the mainshock, but all the aftershocks to the north of the mainshock epicenter are smaller but more numerous M4 events have been recorded. Further, the northwest aftershock cluster exhibits complex faulting on both northwest and northeast planes. Thus the aftershocks, also express a complex pattern of stress release along strike. In addition, some triggered seismicity is being recorded along the Elsinore and San Jacinto faults to the north but significant northward migration of aftershocks has not occurred.

The overall rate of decay of the aftershocks is similar to the rate of decay of a generic California aftershock sequence. Using the available arrival time data, we determine refined 3D Vp and Vp/Vs velocity model. The starting model is based on 1D models from (Vidal et al., 2009). The 3D model shows a strong change in the velocity structure about 15 km north of the mainshock epicenter, with lower crustal velocities and deeper aftershock focal depths to the south. These models have somewhat limited resolution south of the border but are essential for constraining the focal depths of events in this sequence. The first motion focal mechanisms and the waveform based moment tensors of aftershocks exhibit transtensional faulting, consistent with the mainshock faulting.

Additional co-authors on this abstract: I. Méndez, L. Orozco, O. Gálvez, and S. Arregui, Department of Seismology, CICESE, Baja California, Mexico, and K. FELZER, D. GIVEN, L.M. JONES, USGS, Pasadena, CA