The mechanisms of earthquakes and faulting in the southern Gulf of California

Accurate earthquake locations and their focal mechanisms can illuminate the distribution and mode of deformation at rifted continental margins. The Pacific- North America (Pa-NA) plate boundary within the Gulf of California (GoC) provides an excellent opportunity to explore the evolution and kinematics of rifting, as continental extension in the north transitions to seafloor spreading in the south. From October 2005 to October 2006, an array of eight four-component ocean-bottom seismo- graphs deployed in the GoC recorded seismicity as part of the Sea of Cortez Ocean- Bottom Array (SCOOBA) experiment. By combining the data with those from the onshore Network of Autonomously Recording Seismographs (NARS)-Baja array, we detect and locate ~700 earthquakes (M_w 2.2-6.6) mainly along the northwest-south- east-striking transform faults that delineate the plate boundary. For 36 events (M_w 3.5- 6.6) with high signal-to-noise ratio in a long-period (10-20 s) band, we determine deviatoric moment tensors and associated double-couple focal mechanisms by regional waveform inversion. Many focal mechanisms are consistent with right-lateral strike-slip faulting along the Pa-NA transform fault system, which suggests that the transform faults primarily accommodate seismic deformation within the gulf. In addition, we capture a swarm of events on Baja California along the right-lateral northwest-southeast-striking Las Viboras-El Azufre fault, which may be related to ongoing geothermal activity and volcanic deformation within the peninsula. The combination of high-resolution earth- quake locations and focal mechanisms improves our understanding of the distribution of seismic deformation within the greater extensional zone in the southern GoC.