STRONG-MOTION DATA COLLECTED IN BAJA CALIFORNIA DURING THE EL MAYOR-CUCAPAH EARTHQUAKE OF 4 APRIL 2010 (MW 7.2): PRELIMINARY RESULTS

The El Mayor-Cucapah earthquake of 4 April 2010 occurred at 15:40 local time in Baja California (22:40 UTC). Since February 1892, when an earthquake of similar magnitude occurred at the northwestern end of the Laguna Salada fault, no other earthquake of this size had occurred in the area. The April 2010 earthquake had its epicenter at a point located slightly east of the zone at which the sierras El Mayor and Cucapah join together. Its effects were felt at more than 300 miles from the epicenter.

In this study, we analyze the strong motion data recorded by thirteen stations of a seismic network that is operated by CICESE in the north Baja California region. Our dataset is constituted by accelerograms produced by the main event at distances that are between 12 and 140 km from its epicenter. Six out of the thirteen triggered stations are located on the sediments of the Mexicali Valley, at less than 35 kilometers from the main shock epicenter; the other stations recorded the earthquake on granitic rocks of the peninsular ranges of Baja California.

In this paper, we present the epicenter of the main event that was determined using only the P-wave arrival times from records of the strong- and weak-motion stations that were closer to the epicenter. In the hypocenter location process, the determination of the focal depth was poorly constrained by data. However, a tendency of the location program to put the focus at a shallow depth was clearly noted. Several runs of the epicenter location program were performed for fixed values of depth. The results show that depths between 2 and 10 km produce nearly the same horizontal coordinates, but the lower time residuals are observed in association with the shallower-depth solutions.

Regarding strong motion parameters, the recorded peak ground acceleration, velocity and displacement values were by far higher at the sediment sites (12 to 35 km distance) than at the rock sites (60 to 140 km distance). On sediments, the recorded peak accelerations varied from 145 to 799 gals, while on the rock sites the peak accelerations were in the range 13 to 73 gals. The peak ground velocity and displacement values recorded on sediment sites are between 14 and 61 cm/sec and 9 and 52 cm, respectively, with the larger values observed on the horizontal components of motion.

In addition to the above strong motion characteristics, aspects of the earthquake like ground motion spectra and preliminary estimations of the seismic energy radiation and stress drop, based on the locally recorded strong motion recordings, will also be presented.