Abstract
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 ruptures. 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 fault.
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
N130E, but dip in opposite directions. The earthquake was initiated
on the connecting extensional fault and 15s 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.
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