Paleoseismic studies of the San Francisco Peninsula segment of the San Andreas Fault zone near Woodside, California

Abstract

A late Holocene slip rate is proposed for the San Francisco Peninsula segment of the San Andreas fault zone at the Filoli Center, located between the town of Woodside and Upper Crystal Springs Reservoir, California. At this site the primary trace of the San Andreas fault zone traverses N35°W across the head of a large alluvial fan, offsetting buried stream channel deposits. One deposit, with a minimum calibrated accelerator mass spectrometry (AMS) radiocarbon age of 2070±120 years B.P., is offset 30±2 m, yielding an average geologic slip rate of 17±4 mm yr−1 for the late Holocene. A stratigraphically lower channel sequence, with a minimum calibrated AMS radiocarbon age of 330±200 years B.P., is offset 4.1±0.5 m. We interpret that this older sequence records both the penultimate earthquake on the peninsula segment, accompanied by 1.6±0.7 m of dextral slip, and the ∼2.5 m of slip that occurred in 1906. On the basis of radiocarbon analyses and historical descriptions we propose that the pre‐1906 earthquake occurred in June 1838. Our interpretations suggest that the San Francisco Peninsula segment of the San Andreas fault zone may rupture in earthquakes of at least two magnitudes: (1) a large‐magnitude (≈Mw 8) event with a recurrence of ∼250–320 years that involves the North Coast, San Francisco Peninsula, and Santa Cruz Mountains segments, as occurred in 1906 and (2) a Mw 7.0–7.4, 1838‐type event that ruptures only the San Francisco Peninsula segment. The apparent difference in slip rate of 7 ± 7 mm ‐yr−1 between the North Coast segment (24±3 mm yr−1) and the peninsula segment (17±4 mm yr−1) suggests a possible slip rate gradient on the northern California section of the San Andreas fault.