SCEC Award Number 25329 View PDF
Proposal Category Individual Research Project (Single Investigator / Institution)
Proposal Title Using foraminifera to identify co-seismic subsidence along the San Andreas Fault beneath Tomales Bay
Investigator(s)
Name Organization
Alexander Simms University of California, Santa Barbara
SCEC Milestones A1-3, A3-4 SCEC Groups Geology, SDOT, PBS
Report Due Date 03/15/2026 Date Report Submitted 01/30/2026
Project Abstract
 As much as 43 cm of co-seismic subsidence occurred in upper Tomales Bay, California during the 1906 San Francisco Earthquake. This nearly 0.5 m of subsidence suggests the stratigraphy within the bay may contain a record of past earthquakes along the northern San Andreas Fault. As part of earlier work, we created a foraminifera-based transfer function for identifying past periods of co-seismic subsidence and collected 36 cores in Tomales Bay. The purpose of this work was to foraminifera and sedimentological changes within the upper reaches of Tomales Bay to produce a record of past earthquakes along the northern San Andreas Fault. To accomplish these objectives, we proposed to 1.) create a foraminifera transfer function for Tomales Bay using the distribution of extant foraminifera, 2.) apply the transfer function across sedimentological contacts in cores to test for submergence and 3.) acquire radiocarbon ages across potential co-seismic subsidence events. From our work we have 1.) created a foraminiferal transfer function, 2.) identified 5 possible co-seismic, and 3.) used a hierarchical cluster model to compare our ages along with other studies along the northern San Andreas Fault to provide a better statistical evaluation of through going ruptures along the San Andreas Fault. We confirm an ~300 year return interval for earthquakes since 1300 AD but find a potential change in earthquake frequency prior to that with a possible fourth earthquake at 1200 AD.
This project explicitly addresses SCEC Science Milestones A1-3 and A3-4.
Intellectual Merit We provided a longer record of paleoearthquakes along the northern San Andreas Fault dating back to ~1300 BC. We also applied a new technique for comparing earthquake histories from diverse earthquake records that provides a more quantitative way of determining if the events recorded at multiple spots represent the same earthquake. This new technique provides a test for larger through-going earthquakes.
Broader Impacts This project supported the work of one PhD student as well as giving 3 other graduate students valuable field experience.
Project Participants Claire Divola, Trap Puckette, Lauren Mumby, and Matt Giess
Exemplary Figure Figure 3 from the PDF. It provides a hierarchical cluster model of potential earthquakes across the northern San Andrea Fault.
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