Abrupt along-strike change in tectonic style: San Andreas fault zone, San Francisco Peninsula

Seismicity and high-resolution aeromagnetic data are used to define an abru pt 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 o ffshore from the Golden Gate. This 100-km-long section of the San Andreas f ault includes the hypocenter of the M-w = 7.8 1906 San Francisco earthquake as well as the highest level of persistent microseismicity along that simi lar to 470-km-long rupture. We define two distinct zones of deformation alo ng this stretch of the fault using well-constrained relocations of all post -1969 earthquakes based a joint one-dimensional velocity/hypocenter inversi on and a redetermination of focal mechanisms. The southern zone is characte rized by thrust- and reverse-faulting focal mechanisms with NE trending P a xes that indicate "fault-normal" compression in 7- to 10-km-wide zones of d eformation on both sides of the San Andreas fault. A 1- to 2-km-wide vertic al zone beneath the surface trace of the San Andreas is characterized by it s almost complete lack of seismicity. The compressional deformation is cons istent with the young, high topography of the Santa Cruz Mountains/Coast Ra nges as the San Andreas fault makes a broad restraining left bend (similar to 10 degrees) through the southernmost peninsula. A zone of seismic quiesc ence similar to 15 km long separates this compressional zone to the south f rom a zone of combined normal-faulting and strike-slip-faulting focal mecha nisms (including a M-L = 5.3 earthquake in 1957) on the northernmost penins ula and offshore on the Golden Gate platform. Both linear pseudo-gravity gr adients, calculated from the aeromagnetic data, and seismic reflection data indicate that the San Andreas fault makes an abrupt similar to 3-km right step less than 5 km offshore in this northern zone. A similar right-steppin g (dilatational) geometry is also observed for the subparallel San Gregorio fault offshore. Persistent seismicity and extensional tectonism occur with in the San Andreas right stepover region and at least 15 km along-strike bo th to the SE and NW. The 1906 San Francisco earthquake may have nucleated w ithin 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-w = 7 to 8 strike-slip earthquake along the San Andreas fault: the poten tial for a M approximate to 6 normal-faulting earthquake just 5-8 km west o f San Francisco and a M = 6+ thrust faulting event in the southern peninsul a.