Superficial simplicity of the 2010 El Mayor–Cucapah earthquake of Baja California inMexico

The geometry of faults is usually thought to be more complicated at the surface than at depth and to control the initiation, propagation and arrest of seismic ruptures1–6. The fault system that runs from southern California into Mexico is a simple strike-slip boundary: the west side of California and Mexico moves northwards with respect to the east. However, the Mw 7.2 2010 El Mayor–Cucapah earthquake on this fault system produced a pattern of seismic waves that indicates a far more complex source than slip on a planar strike-slip fault7. Here we use geodetic, remote-sensing and seismological data to reconstruct the fault geometry and history of slip during this earthquake. We find that the earthquake produced a straight 120-km-long fault trace that cut through the Cucapah mountain range and across the Colorado River delta. However, at depth, the fault is made up of two different segments connected by a small extensional fault. Both segments strike N130 E, but dip in opposite directions. The earthquake was initiated on the connecting extensional fault and 15 s later ruptured the two main segments with dominantly strike-slip motion. We show that complexities in the fault geometry at depth explain well the complex pattern of radiated seismic waves. We conclude that the location and detailed characteristics of the earthquake could not have been anticipated on the basis of observations of surface geology alone. The El Mayor–Cucapah earthquake that occurred on 4 April 2010 produced extensive liquefaction in the Colorado River delta area and in the Mexicali and Imperial valleys, and numerous rockfalls occurred in the Sierra Cucapah. This is the largest earthquake to have struck the southern California and northern Baja California, Mexico area since the Mw 7.3 Landers earthquake of 1992 (ref. 8). The GCMT (global centroid moment tensor) of the mainshock reveals a double-couple component corresponding to a scalar moment of 7.28× 1019 Nm (Mw 7.17), with a significant non-double-couple (CLVD) component (2.4× 1019 Nm; Fig. 1; ref. 7). The mainshock occurred where the system of continental parallel right-lateral strike-slip faults including the San Andreas, San Jacinto and Elsinore faults connect with a system of transform faults and active spreading centres in the Gulf of California to the south9 (Fig. 1 and Supplementary Fig. S1). This fault system forms the plate boundary in southern California, where the Pacific plate moves northwestwards with respect to North America at about 46mmyr−1 (Fig. 1, inset). The main active fault recognized in the