THE MOST RECENT LARGE EARTHQUAKE ON THE RODGERS CREEK FAULT, SAN FRANCISCO BAY REGION

The Rodgers Creek fault (RCF) is a principal component of the San Andreas fault system north of San Francisco. No evidence appears in the historical record of a large earthquake on the RCF, implying that the most recent earthquake (MRE) occurred before 1824, when a Franciscan mission was built near the fault at Sonoma, and probably before 1776, when the written record began regionally in San Francisco. The first appearance of nonnative pollen in the stratigraphic record at the Triangle G Ranch study site on the south-central RCF confirms that the MRE occurred before local settlement and the beginning of livestock grazing. Chronological modeling of earthquake age using radiocarbon-dated detrital charcoal from near the top of a faulted alluvial sequence at the site indicates that the MRE occurred no earlier than A.D. 1690 and most likely occurred after A.D. 1715. With these age constraints, we know that the elapsed time since the MRE on the RCF is more than 181 years and less than 315 years and is probably between 229 and 290 years. This elapsed time is similar to published recurrence-interval estimates of 131 to 370 years (preferred value of 230 years) and 136 to 345 years (mean of 205 years), calculated from geologic data and a regional earthquake model, respectively. Importantly, then, the elapsed time may have reached or exceeded the average recurrence time for the fault. The timing of the MRE on the RCF is similar to prehistoric rupture on the north and south segments of the Hayward fault (A.D.1640-1776). This allows the possibility that an earthquake, or closely timed earthquakes, ruptured both the RCF and the Hayward fault across the San Pablo Bay step-over.

A buried channel is offset 2.2 (+1.2, -0.8) m along one side of a pressure ridge at the Triangle G Ranch site. This provides a minimum estimate of right-lateral slip during the MRE at this location. Total slip at the site may be similar to, but is probably greater than, the 2 (+0.3, -0.2) m measured previously at the nearby Beebe Ranch site.