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The role of a keystone fault in triggering the complex El Mayor-Cucapah earthquake rupture

John M. Fletcher, Michael E. Oskin, & Orlando J. Teran

Accepted 2015, SCEC Contribution #1962

Large earthquakes commonly occur on multiple faults as ruptures cascade through complex fault networks. Because the critical stress at failure varies greatly with fault orientation, it is difficult to explain how faults with diverse orientations rupture spontaneously in a single earthquake, or how misoriented faults ever fail in the presence of optimally oriented faults that release tectonic loading at lower thresholds. Here we examine the 2010 Mw 7.2 El Mayor-Cucapah earthquake, which initiated on a normal fault that underpins a complex network of faults. The progenitor normal fault, due to its orientation, was among those that required the greatest differential stress for failure. Other faults with more optimal orientations must have reached critical stress earlier in the interseismic period, but pinning linkages with misoriented faults regulated their slip. This slip regulation maintains faults of diverse orientations at critical stress without destabilizing the network. Regional stress buildup thus continues until a misoriented keystone fault reaches its threshold and subsequent failure spreads spontaneously to other critically stressed faults in the network, producing a large earthquake. This keystone fault hypothesis explains seismogenic failure of severely misoriented faults like the San Andreas and extensional detachments, and allows these to coexist with optimally oriented faults without large variations in their expected frictional strengths.

Fletcher, J. M., Oskin, M. E., & Teran, O. J. (2015). The role of a keystone fault in triggering the complex El Mayor-Cucapah earthquake rupture. Nature, (accepted).