SLIP RATE ON THE SAN ANDREAS FAULT, SAN FRANCISCO PENINSULA, CALIFORNIA

The most recent large earthquake on the San Francisco Peninsula segment of the San Andreas Fault (SAF) was the great Mw 7.8 San Francisco earthquake of April 18, 1906. This event ruptured the North Coast, Peninsula, and Santa Cruz Mountains segments of the SAF. An accurate slip rate for the peninsula segment is important for refining seismic hazards in this highly urbanized region. Two previous slip rate studies on the San Francisco Peninsula yielded differing slip-rate estimates. One study by Hall (1984), at the San Andreas Dam site, estimated an average slip rate of ~12 mm/yr and the other study by Hall et al. (1999) at the Filoli site, estimated an average slip rate of 17±4 mm/yr. We excavated a trench north of these two sites where an abandoned stream channel crosses the SAF between the San Andreas and Lower Crystal Spring reservoirs. Channel abandonment occurred when a new channel cut across the fault. The new, and currently active channel has since been offset in a right-lateral sense. Our trench investigation revealed channel-fill alluvium that was later incised and filled with probable debris flow sediments, and a 2-m wide fault zone in Franciscan Complex bedrock west of the channel-fill deposits. One of the prominent fault strands in the trench breaks nearly all the way to the surface and is probably the strand that ruptured in 1906. The debris-flow deposits contained grass seeds that yielded radiocarbon ages of 910-730 cal. yr BP, 1310-1190 cal. yr BP, and 2340-2150 cal. yr BP. These dates are consistent with their stratigraphic order and indicate the occurrence of multiple debris-flow events. We assume that the age of the youngest debris flow sediments approximate both the abandonment age of the older channel and the initiation of the active channel. A total-station survey of channel morphology together with LiDAR data, allow us to estimate the amount of offset of the active channel. Right-lateral offset of this active channel is between 6–18 m with a preferred estimate of ~13 m. If initiation of this offset is synchronous with abandonment of the older channel, then the SAF at this site shows a late Holocene slip rate between 7–25 mm/yr with a preferred rate of 14–18 mm/yr. This preferred rate is similar to rates determined by earlier studies.