THE NORTHERN SAN-GREGORIO FAULT ZONE - EVIDENCE FOR THE TIMING OF LATE HOLOCENE EARTHQUAKES NEAR SEAL COVE, CALIFORNIA

The San Gregorio fault is the principal active fault west of the San A ndreas fault in central coastal California, yet it remains the largest known fault in the region whose seismogenic potential is not known. I n this study, we integrate traditional paleoseismic and archaeologic i nvestigations to define the location, style, and timing of slip events on the northern San Gregorio fault at a site near Seal Cove in Moss B each, California. The on-land portion of the San Gregorio fault at Sea l Cove is a late-Holocene active dextral slip fault. Trench excavation s revealed a broad zone of faulting, at least 22 m wide, consisting of five Holocene-active strands. These include a single mid-Holocene eas t-vergent reverse fault and four late-Holocene near-vertical strike-sl ip faults. The most recent event occurred after the deposition of a na tive Californian cooking hearth dated A.D. 1270 to A.D. 1400, but prio r to the arrival of Spanish missionaries ca. 1775. The penultimate eve nt at the site is less well constrained but appears to have occurred b etween A.D. 620 and A.D. 1400. The penultimate event was associated wi th horizontal displacement on the order of 3 m, based on reconstructio n of a thrust wedge within the fault zone. The geometry of midden depo sits shows a 5 (-2, +6) m deflection along the projection of faults as sociated with the most recent event (MRE). All or part of this deflect ion may be associated with the MRE. These displacements are consistent with M-W 7 to 7 1/4 earthquakes and show that the San Gregorio fault is an active seismogenic source that should be considered in seismic h azard assessments in the San Francisco Bay area.