SCEC Award Number 25188 View PDF
Proposal Category Collaborative Research Project (Multiple Investigators / Institutions)
Proposal Title Numerical Simulations of the Change in Stress-State Inboard of the Mendocino Triple Junction
Investigator(s)
Name Organization
Xiaochuan Tian University of California, Davis Magali Billen University of California, Davis Michael Oskin University of California, Davis
SCEC Milestones B1-1, B3-2, C1-1, C2-1, C2-2, C3-1 SCEC Groups SDOT, CEM, RC
Report Due Date 03/15/2026 Date Report Submitted 04/27/2026
Project Abstract
 The San Andreas Fault (SAF) grows by continuous northward migration of the Mendocino Triple Junction (MTJ), where the Cascadia subduction zone and the SAF meet. Topography and faulting styles are drastically different across the latitude of MTJ from north to south. How the passage of MTJ allows this transition to happen is not clear. Here, we hypothesize that the strength of the subduction plate boundary shear zone is strain-rate dependent. Once ridge-trench collision causes slab detachments, shear deformation at the interface will slow down, which allows the subduction shear zone to become an inactive and partially coupled interface. This interface can be strong enough to transmit force from the northward moving Pacific plate to activate the strike-slip SAF system. Building upon previous subduction modeling studies that include ridge-trench collision and slab detachment, we will further develop 3D models that allow both the subducting plate and upper plate to deform in response to the changes in stress-state as the triple junction passes by. Constraints from the community rheology and thermal models will help guide our model setup. Comparing model results with decades of multi-disciplinary observations, we aim to constrain the change in stress state in the overriding plate, and stress acting across emerging fault zones as the MTJ moves northward.
SCEC Community Models Used Community Thermal Model (CTM), Community Rheology Model (CRM), Community Stress Model (CSM), Community Geodetic Model (CGM), Community Fault Model (CFM)
Usage Description The CTM and CRM help us set up our model. We will use
the CSM, CGM and CFM to constrain our model results.
Intellectual Merit Using a statistical analysis of global subduction zone earthquakes we have found that the oblique motion of the Juan de Fuca plate north of the Mendocino Triple Junction (MTJ) is expected to be partially partitioned between oblique slip on the megathrust and trench parallel strain in the upper plate; complete partitioning is not expected. This result provides a key constraint on the material properties for the plate boundary shear zone in our geodynamical model of the MTJ relative to the material properties of the overriding plate: this constraint will help to constrain the magnitude of stress in the upper plate.
Broader Impacts In addition to providing a key constraint for the geodynamic models of the MTJ, the global analysis provides insight into the partitioning of slip along most major subduction zones. The conclusion that partitioning occurs regardless of relative strength and for all obliquity angles means that estimates of cumulative slip deficit on the megathrust should not use the full plate convergence, but rather scale this for the observed slip partitioning.
Project Participants Dr. Tian carried out the primary work on this project with guidance from Dr. Billen and Dr. Oskin. Dr. Tian also collaborated with CIG scientists for development of the initial MTJ model geometry.
Exemplary Figure Figure 3: 3D perspective view of the plate geometries for the Mendocino Triple Junction
geodynamic model. Left: The blue plates are portions of the modeled Pacific and Juan
de Fuca Plates: the Juan de Fuca plate is connected to a subducted slab. The orange
plate is a portion of the North America plate. Right: Cross sections showing only the
oceanic plate thermal structure.
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