A RECORD OF EARTHQUAKES ALONG THE NORTHERN SAN ANDREAS FAULT PRESERVED IN THE SEDIMENTS OF 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 25 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 of relative sea level 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 mouthbar 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 main candidate subsidence event was constrained using radiocarbon dating to within the last 800-1300 years, and paleo-environmental changes generated by the foraminifera transfer function were used to solidify the event. This event was compared to similar records found in neighboring Olema and Bolinas Lagoon thought to represent past earthquakes along the northern San Andreas Fault. Our new foraminiferal transfer function provides a valuable new tool for the reconstruction of relative sea level changes along the California coast, as well as allow us to add to existing records of past earthquakes along the Northern San Andreas Fault.