Earthquake Rupture Complexity Evidence from Field Observations (Invited)

Field observations provide strong evidence for four intriguing aspects of rupture process complexity for the 4 April 2010 El Mayor - Cucapah - Indiviso earthquake. First, the southern “fork” of the rupture exhibits two splays just to the northwest of the event epicenter, both with nearly pure right-lateral faulting. Teleseismic source modeling by others indicates normal slip on deep fault surface preceded the shallow strike-slip faulting, and our field observations from the fork area indicate that the earliest phase of strike-slip faulting did not involve a significant normal faulting component. Second, as rupture propagated to the northwest along the Pescadores fault, slip ended abruptly on this fault and transferred across a complex zone to the Borrego fault (although the Pescadores fault continues farther and would have seemed an easier route to follow). This stepover is called the Puerta Accommodation Zone (PAZ) which extends 11 km along-strike within an elevated portion of the Cucapah massif and accommodates a left step (transpressional) that measures less than 2 km across-strike. Though partly obscured by rockfalls in the steep terrain here, only discontinuous faulting of up to one meter reached the ground surface, yet imagery differencing by others indicates several meters of continuous subsurface slip throughout this section. This 11 km region of reduced and discontinuous slip is one of the longest surface rupture jumps ever observed, with lengthy and continuous surface faulting on either side. The SE end of the stepover coincides spatially with the Canon Rojo embayment where the M7.2 1892 surface rupture propagated onto a second fault forming an abrupt corner in its surface trace. Hence, the surface rupture and slip distribution pattern in 2010 appears to have been influenced by stress changes induced by the 1892 event. Thirdly, from the NW end of the transition section, the Borrego fault continues to the NNW with strong east-down slip obliquity. Along this same reach of fault, the Laguna Salada fault was re-activated with minor west side down slip. Along the Paso Inferior accommodation zone, splays trending NNW to N had strong east-down slip, with right-lateral oblique motion. It was along this portion of the rupture that an eyewitness observed the rupture propagating from N to S, instead of in the expected S to N direction. Back-rupture may have resulted from triggered slip having raced ahead of the slower main rupture front, or may indicate a place where contiguous deep rupture ran out in front of shallow slip, or may result from the involvement here of the detachment fault system. Finally, in the northwestern portion of the rupture, from Paso Superior to the northwest of Cerro Centinela, a pattern of NE - SW oriented “cross faults” and N-S faults broke at the surface, in a manner similar to rupture that occurred in the Elmore Ranch Fault Zone during the Superstition Hills earthquake sequence of 1987. Unlike the Elmore Ranch event, however, there was no known main shock sub-event or any large aftershock that can be readily associated with the extensive cross-faulting that is observed in this case.