SCEC Award Number 25062 View PDF
Proposal Category Collaborative Research Project (Multiple Investigators / Institutions)
Proposal Title Identifying Potential Seismogenic Structures in the Northern San Andreas Plate Boundary Fault System
Investigator(s)
Name Organization
Matthew Herman California State University, Bakersfield Kevin Furlong Pennsylvania State University
SCEC Milestones A1-1, A1-3, A2-2, A3-1, A3-3, A3-5, B2-1, B3-1 SCEC Groups PBS, SDOT, Geodesy
Report Due Date 03/15/2026 Date Report Submitted 03/30/2026
Project Abstract
 The San Andreas Plate Boundary (SAPB) between the North American and Pacific plates spans almost the entire extent of California. It has reached a mature stage in central and southern California, with a primary crustal fault (the San Andreas Fault; SAF) overlying a lithospheric scale shear zone. In contrast, the SAPB is still evolving from the San Francisco Bay Area to its termination at the Mendocino triple junction (MTJ). As the MTJ migrates northward, the SAPB is lengthening and forming at a rate of 40 mm/yr in the wake of the retreating subduction system. How this segment of the SAPB evolves, leading to the development of seismogenic crustal faults, is the underlying motivation for this proposed research. There are two primary goals for this project: (a) define the primary active plate boundary structures in the SAPB system throughout northern California; and (b) assess the implications for earthquake behavior and interactions among these structures. First, we will generate a 3-D geological structure (including active faults, elastic properties, and rheologies) for the northern SAPB incorporating new geophysical and geological observations. We will apply these results to physics-based numerical models of the SAPB system in northern California to constrain the present-day interseismic coupling state of the identified fault structures, as well as the signal of this coupling in surface-based observations. Finally, we will use these models to test coseismic slip and stress transfer scenarios, particularly around the buried, sub-horizontal detachment extending from the SAF to the lithospheric shear zone further east.
Intellectual Merit The goal of this project was to improve understanding of the seismogenic potential of plate boundary structures at the Mendocino triple junction, and along the northern San Andreas plate boundary. Our first objective was to define the primary structures, identifying features that are buried or offshore. Our second objective was to develop models to assess earthquake behavior, inter-earthquake stress accumulation, and triggering interactions. This project aimed at filling critical data gaps in the tectonic evolution of the San Andreas plate boundary system that define its fault structure, the deformational nature of these structures, patterns of inter-seismic loading, and thermal structure.
Broader Impacts This models developed in this project have led to enhanced capabilities for studying the seismotectonics of the San Andreas plate boundary evolution and earthquake potential at California State University, Bakersfield, a Hispanic Serving Institution. These materials have been incorporated into course curricula and undergraduate research projects in the Department of Geological Sciences. Incorporating better constraints on fault structures in northern California will lead to be better constraints on the range of potential earthquake hazards in the region.
Project Participants Matthew Herman (California State University, Bakersfield)
Kevin Furlong (Pennsylvania State University)
Exemplary Figure Figure 1. Seismotectonic map of the northern San Andreas plate boundary. The San Andreas fault (red line) south of Point Arena divides the Salinian block (pink) from the Franciscan terrane (blue). The Salinian block is inferred to end north of Point Arena at the Navarro discontinuity. Northwest of Point Arena, the location of the Pioneer fragment constrained by seismic tomography imaging. The total velocity between the Pacific plate and Great Valley block is 40 mm/yr. The location of the lithospheric-scale transform plate boundary, as constrained by interseismic GPS velocities, is indicated by the dashed line.
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