Abrupt along‐strike change in tectonic style: San Andreas Fault zone, San Francisco Peninsula

Abstract

Seismicity and high‐resolution aeromagnetic data are used to define an abrupt change from compressional to extensional tectonism within a 10‐ to 15‐km‐wide zone along the San Andreas fault on the San Francisco Peninsula and offshore from the Golden Gate. This 100‐km‐long section of the San Andreas fault includes the hypocenter of the Mw = 7.8 1906 San Francisco earthquake as well as the highest level of persistent microseismicity along that ∼470‐km‐long rupture. We define two distinct zones of deformation along this stretch of the fault using well‐constrained relocations of all post‐1969 earthquakes based a joint one‐dimensional velocity/hypocenter inversion and a redetermination of focal mechanisms. The southern zone is characterized by thrust‐ and reverse‐faulting focal mechanisms with NE trending P axes that indicate “fault‐normal” compression in 7‐ to 10‐km‐wide zones of deformation on both sides of the San Andreas fault. A 1‐ to 2‐km‐wide vertical zone beneath the surface trace of the San Andreas is characterized by its almost complete lack of seismicity. The compressional deformation is consistent with the young, high topography of the Santa Cruz Mountains/Coast Ranges as the San Andreas fault makes a broad restraining left bend (∼10°) through the southernmost peninsula. A zone of seismic quiescence ∼15 km long separates this compressional zone to the south from a zone of combined normal‐faulting and strike‐slip‐faulting focal mechanisms (including a ML = 5.3 earthquake in 1957) on the northernmost peninsula and offshore on the Golden Gate platform. Both linear pseudogravity gradients, calculated from the aeromagnetic data, and seismic reflection data indicate that the San Andreas fault makes an abrupt ∼3‐km right step less than 5 km offshore in this northern zone. A similar right‐stepping (dilatational) geometry is also observed for the subparallel San Gregorio fault offshore. Persistent seismicity and extensional tectonism occur within the San Andreas right stepover region and at least 15 km along‐strike both to the SE and NW. The 1906 San Francisco earthquake may have nucleated within the San Andreas right stepover, which may help explain the bilateral nature of rupture of this event. Our analysis suggests two seismic hazards for the San Francisco Peninsula in addition to the hazard associated with a M = 7 to 8 strike‐slip earthquake along the San Andreas fault: the potential for a M ≈ 6 normal‐faulting earthquake just 5–8 km west of San Francisco and a M = 6+ thrust faulting event in the southern peninsula.