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

The 2010 Mw 7.2 El Mayor-Cucapah earthquake in Baja California, Mexico activated slip on multiple faults of diverse orientations(1,2), which is commonly the case for large earthquakes(3-6). The critical stress level for fault failure depends on fault orientation and is lowest for optimally oriented faults positioned approximately 30 degrees to the greatest principal compressive stress(7). Yet, misoriented faults whose positioning is not conducive to rupture are also common(8,9). Here we use stress inversions of surface displacement and seismic data to show that the El Mayor-Cucapah earthquake initiated on a fault that, owing to its orientation, was among those that required the greatest stress for failure. Although other optimally oriented faults must have reached critical stress earlier in the interseismic period, Coulomb stress modelling shows that slip on these faults was initially muted because they were pinned, held in place by misoriented faults that helped regulate their slip. In this way, faults of diverse orientations could be maintained at critical stress without destabilizing the network. We propose that regional stress build-up continues until a misoriented keystone fault reaches its threshold and its failure then spreads spontaneously across the network in a large earthquake. Our keystone fault hypothesis explains seismogenic failure of severely misoriented faults such as the San Andreas fault and the entire class of low-angle normal faults.