A Record of Earthquakes along the Northern San Andreas Fault Preserved in the Sediments of Tomales Bay

Claire Divola, Alexander R. Simms, & Ed Garrett

Submitted February 2, 2026, SCEC Contribution #15014

The Northern San Andreas Fault threatens more than seven million people, and its 1906 earthquake caused widespread damage and induced subsidence in nearby estuaries, including Tomales Bay. To determine if Tomales Bay’s marshes hold a record of past San Andreas Fault Zone earthquakes, we collected 25 sediment cores and surveyed the elevational distribution of modern foraminifera assemblages. Fossil assemblages were calibrated against modern distributions using a Bayesian transfer function to reconstruct past environments and detect and quantify the magnitude of abrupt vertical changes indicative of potential co-seismic subsidence. Nine cores contain sharp lithologic contacts interpreted as candidate co-seismic submergence surfaces constrained by radiocarbon dating to approximately A.D. 1175-1223, A.D. 729-866, A.D. 572-687, A.D. 56 - 277 B.C., and 1294-1387 B.C. Paleoenvironmental reconstructions using the transfer function support the interpretation that two of these surfaces (2 and 4) represent abrupt subsidence events; the other three (1, 3 and 5) are less conclusive. A hierarchical clustering model is used to compare Tomales Bay events to other paleoseismic records on the North Coast Segment of the San Andreas Fault. The events at Tomales Bay add to previously documented earthquakes along the North Coast segment, suggesting that throughgoing ruptures with approximately 300-year intervals started around A.D. 1300, but occurred at different frequencies before that. We also find potential for a throughgoing rupture around A.D. 1200, suggesting a shorter interval between successive ruptures. Our 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. (2026). A Record of Earthquakes along the Northern San Andreas Fault Preserved in the Sediments of Tomales Bay. Earth Science Reviews, (submitted).