USING FORAMINIFERA TO IDENTIFY EARTHQUAKES ALONG THE NORTHERN SAN ANDREAS FAULT, TOMALES BAY, CALIFORNIA

During the 1906 San Francisco Earthquake, portions of the marshes in upper Tomales Bay, which floods part of the San Andreas Fault valley, underwent co-seismic subsidence. Thus, the deposits of these marshes may hold a record of past earthquakes along the Northern San Andreas Fault. In this study, we collected 36 sediment cores and surveyed the elevational zonation of modern foraminifera within upper Tomales Bay to search for past subsidence events within the sedimentary record. The modern foraminifera were compared to fossil assemblages from sediment cores to create a paleo-environmental reconstruction using a Bayesian transfer function (Cahill et al., 2016). Six different sedimentary facies within the cores were identified including those representing bay deposits, marsh deposits, transitional deposits, terrestrial deposits, fluvial deposits, and the mouth bar of the Lagunitas Creek bayhead delta. In several of the cores we identified a sharp contact between underlying marsh deposits capped by bay deposits, which may represent co-seismic subsidence of the delta during a past earthquake. The age of the candidate subsidence event was constrained using radiocarbon dating to within the last 800-1300 years, and paleo-environmental changes generated by the transfer function were used to solidify the event. This event may correlate with a similar record found in neighboring Bolinas Lagoon thought to represent a past earthquake along the northern San Andreas Fault. The use of sediment cores and foraminifera to identify coseismic subsidence will allow us to add to existing records of past earthquakes along the Northern San Andreas Fault and provide a more complete picture of past earthquake periodicity within the Northern San Andreas Fault Zone.