A Record of Earthquakes along the Northern San Andreas Fault from Subsidence Events within Tomales Bay, California

Claire Divola, Alexander R. Simms, & Ed Garrett

Submitted September 7, 2025, SCEC Contribution #14601, 2025 SCEC Annual Meeting Poster #TBD

During the 1906 San Francisco earthquake, portions of the marshes in upper Tomales Bay, which floods part of the San Andreas Fault valley, experienced coseismic 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 distribution of modern foraminifera to establish a reference dataset. Fossil assemblages were compared to modern distributions using a Bayesian transfer function to reconstruct past environments and detect and quantify the magnitude of abrupt vertical changes indicative of subsidence. We identified seven sedimentary facies across the cores, including bay, marsh, levee, terrestrial, fluvial, and mouthbar deposits of the Lagunitas creek bayhead delta. Several cores contain sharp lithologic contacts that we interpret as candidate coseismic submergence surfaces constrained by radiocarbon dating to approximately A.D. 1600, 1300, 1000, and 650, as well as 100 and 1350 B.C. Paleoenvironmental reconstructions created using the transfer function support the interpretation that these surfaces represent abrupt subsidence events. These events align with previously documented earthquakes along the North Coast segment, suggesting that throughgoing ruptures began recurring at intervals of approximately 300 years starting around A.D. 650. This study demonstrates the effectiveness of combining foraminiferal transfer functions with sediment core analysis to identify past coseismic subsidence. The results expand the earthquake chronology for the northern San Andreas Fault and contribute to a more complete understanding of its long-term rupture behavior.

Citation
Divola, C., Simms, A. R., & Garrett, E. (2025, 09). A Record of Earthquakes along the Northern San Andreas Fault from Subsidence Events within Tomales Bay, California. Poster Presentation at 2025 SCEC Annual Meeting.

Related Projects & Working Groups
Earthquake Geology