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Superficial simplicity of the 2010 El Mayor-Cucapah earthquake of Baja, California in Mexico

Shengji Wei, Eric J. Fielding, Susie LePrince, Anthony Sladen, Jean-Philippe Avouac, Donald V. Helmberger, Egill Hauksson, Risheng Chu, Mark Simons, Kenneth W. Hudnut, Thomas A. Herring, & Richard Briggs

Published 2011, SCEC Contribution #1622

On April 4, 2010, the Mw7.2 El Mayor-Cucapah (Baja, California) earthquake ruptured several faults. This quake started as a normal event, with a mechanism of 355°/45°/-73° (strike/dip/rake), which is supported by the inversion of the first 15 seconds of teleseismic body wave. The following rupture turned into a bilateral transtensional slip on two anti-dipping major faults. The eastward dipping fault to the north of the epicenter has a strike of about 312° and an averaged dip of about 75°, extended about 55km as indicating by the SPOT image. While the west dipping fault extended about almost the same length(~60km) to the south of the epicenter with a strike of about 131° and a dip of about 60°. The sub-pixel correlation of SPOT 2.5 m panchromatic images indicates some detail of rupture such as two kinks about 20 and 45 km to the north of the epicenter, and an average of 2.5m horizontal offset is obtained. Towards southeast of the epicenter, the SAR azimuth offset of ALOS PALSAR track 211 reveals offsets of up to 3 meters ~60 km track with no known surface fault. The joint inversion of the optical, InSAR data sets, the California PBO GPS stations and teleseismic body waves(P&SH) shows two large asperities on the north major segment, which centered about 15 and 40 km away to the north of the epicenter with depth shallower than 10km. Aftershocks relocated with Double Difference display good anti-correlation with the asperity to the north end of the rutpure, where the quakes are well recorded by TriNet station. The joint inversion shows a moment of about 9.9e26 dyne.cm (Mw~7.25) while seismic body wave inversion prefers 7.2e26(Mw~7.17) dyne.cm. The almost even distribution of moment on the westward dipping fault and the eastwards dipping fault(~Mw7.1) produces a large non-Double-Couple component to a CMT solution as reported.

Wei, S., Fielding, E. J., LePrince, S., Sladen, A., Avouac, J., Helmberger, D. V., Hauksson, E., Chu, R., Simons, M., Hudnut, K. W., Herring, T. A., & Briggs, R. (2011). Superficial simplicity of the 2010 El Mayor-Cucapah earthquake of Baja, California in Mexico. Nature GeoScience,. doi: 10.1038/NGEO1213.