The 2010 Mw7.2 El Mayor-Cucapah Earthquake Sequence, Baja California, Mexico and Southernmost California, USA: Active Seismotect

The 2010 Mw7.2 El Mayor-Cucapah Earthquake Sequence, Baja California, Mexico and Southernmost California, USA: Active Seismotectonics along the Mexican Pacific Margin Map of the relocated hypocenters of foreshocks, mainshock, and aftershocks and 2009 background seismicity. The mainshock epicenter is indicated by a star. Foreshocks are shown as blue open circles beneath the mainshock star. The locations of the Vp cross section are indicated by the A-A' (includes focal mechanism of mainshock).
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The El Mayor-Cucapah earthquake 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 Mw7.2 main- shock that occurred on the 4th 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 aftershock zone extends for 120 km from the south end of the Elsinore fault zone at the US-Mexico border almost to the northern tip of the Gulf of California. The waveform-relocated aftershocks form two abutting clusters, of about equal length of 50 km each, as well as a 10 km north-south after- shock zone just north of the epicenter of the mainshock. Even though the Baja California 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 main- shock, and only one M5.7 aftershock but numerous smaller aftershocks to the north. 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. The overall rate of decay of the aftershocks is similar to the rate of decay of a generic California aftershock sequence. In addition, some triggered seismicity was recorded along the Elsinore and San Jacinto faults to the north but significant northward migration of aftershocks has not occurred. The synthesis of the El Mayor-Cucapah sequence reveals transtensional regional tectonics, including the westward growth of the Mexicali Valley as well as how Pacific North America plate motion is transferred from the Gulf of California in the south into the southernmost San Andreas fault system to the north.
</p><p>References
</p><p>Hauksson, E., J. Stock, K. Hutton, W. Yang, A. Vidal, and H. Kanamori, The 2010 Mw7.2 El Mayor-Cucapah Earthquake Sequence, Baja California, Mexico and Southernmost California, USA: Active Seismotectonics Along the Mexican Pacific Margin, Submitted to Pure Appl. Geophys. Topical Issue: Geodynamics of the Mexican Pacific Margin, 10 July 2010
</p><p>Acknowledgements: This research was supported by the USGS, SCEC, and CICESE.</p>

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