PALEOSEISMIC EXCAVATIONS ACROSS THE SAN ANDREAS FAULT ON THE SAN FRANCISCO PENINSULA, WOODSIDE, CALIFORNIA

The Peninsula section of the San Andreas Fault (PSAF) last ruptured as part of the 435 to 470-km-long surface rupture associated with the 1906 San Francisco earthquake. During the years since that historic earthquake, the San Francisco Bay area has developed at a rapid pace, and the PSAF is among the most significant seismic hazards in the region due to its close proximity (within 10 km) to downtown San Francisco and to infrastructure crucial to the technology centers of Silicon Valley. We excavated four paleoseismic trenches across the PSAF at the Filoli Estate in Woodside, CA, to constrain the ages of prehistoric earthquakes along this section of the fault to provide a better understanding of seismic hazard. Our excavations exposed a stratigraphic section of faulted fluvial (channel and overbank), paludal, and colluvial strata overlain by up to about one meter of unfaulted sediments. 83 Radiocarbon samples collected from the exposed sediments yield ages that range from modern (post-bomb) near the top of the exposure on samples collected from unfaulted sediments, to approximately 2000 years near the base of the exposure. Within the approximately 15-m-wide fault zone, we documented several faults that are overlain by unfaulted, post-MRE sediments. We interpret these youngest faults to be associated with the 1906 surface rupture. Most samples collected from the unfaulted sediments yield calibrated radiocarbon age ranges that extend into the 20th century, consistent with our interpretation that this event horizon represents the 1906 earthquake. The trenches exposed evidence, including fault terminations and folded strata, for at least four earlier earthquakes. Radiocarbon analyses of multiple samples collected above and below each event horizon constrain the ages of these prehistoric earthquakes. Our current OxCal-modeled earthquake age ranges for the four prehistoric events are 1290-1360 CE, 1060-1160 CE, 910-1020 CE, and pre-560 CE. These ages suggest a long interval of quiescence prior to the 1906 earthquake, and further suggest that earthquake recurrence intervals along the PSAF are irregular, with three earthquakes occurring within a 450-year period between 910 and 1360 CE, followed by an interval of about 550-620 years with no surface-rupturing earthquake occurring at this site.