Earthquake source mechanisms and transform fault tectonics in the Gulf of California

Abstract

We have determined the source parameters of 19 earthquakes in the Gulf of California from an inversion of long‐period P and SH waveforms. Fifteen of the earthquakes, on transforms between the Cerro Prieto fault and the Tamayo Fracture Zone, are characterized by right‐lateral strike‐slip faulting, and all but one of these likely reflect the relative motion between the North American and Pacific plates. Slip vectors from these events constitute an improved data set for determination of North American‐Pacific motion. Most centroid depths are poorly resolved because of trade‐offs between depth and source time function. Both the ranges in acceptable centroid depths and the average fault width estimated from cumulative moment release during this century are compatible with the hypothesis that seismic slip does not extend below about 10‐km depth, or the approximate depth of the nominal 800°C isotherm, along the principal oceanic transforms of the gulf. Two normal‐faulting earthquakes during a swarm in the northern gulf have centroid depths of 6 ± 3 and 3 ± 2 km, consistent with earthquake centroid depths in other settings (Arctic, northern Red Sea) where plate separation has carried continental rifting nearly to completion. A thrust‐faulting earthquake near the Tres Marias escarpment has a centroid depth of 11 ± 2 km and probably reflects relative motion between the Rivera and North American plates. The transition from oceanic transform faulting to continental transform faulting in the northern gulf is marked by a broadening of the zone of deformation and a 10° change in the strike of the principal fault accommodating plate motion. Fault kinematics in the transition zone may be understood in terms of an unstable FFF triple junction, with the Agua Bianca and San Miguel faults acting to transfer a portion of the plate motion to faults west of the San Andreas system and with the Delfin and Wagner basins forming as a direct result of slip on the three fault systems meeting in the junction region.