Seismic Imaging of the Mendocino Triple Junction: Defining the Geodynamics of an Evolving Plate Boundary Transition

Kevin Furlong, Matthew Herman, & Kirsty McKenzie

Submitted August 30, 2026, SCEC Contribution #15414, 2026 SCEC Annual Meeting Poster #TBD

The Mendocino Triple Junction (MTJ) demarcates the fundamental change along the North American plate boundary from subduction (Cascadia) to translation (San Andreas). This plate boundary transition zone hosts the initiation of the lithospheric shear zone that develops into the San Andreas plate boundary system. It is one of the most seismogenic regions of North America and a location of abrupt changes in topography and relief. Over the past several decades our understanding of the tectonics intrinsic to this transition has advanced, but key questions about the structure and deformational processes at the MTJ remain. Our regional tomography of the MTJ region (Furlong et al., 2024) allows us to better constrain the crust-upper mantle structure and its relationship to the abundant seismicity, improving our understanding of the main controls on San Andreas plate boundary formation. The high-resolution of the tomography (3 km x 3 km x 0.5 km) allows us to clearly differentiate among the 3 primary components of the plate boundary system - the subducting Gorda Slab, the North American crust (composed of the Franciscan, Klamath, and Great Valley terranes), and the Pacific plate (including a fragment that extends inland under the North American crust). We also image the upwelled mantle within the slab window. Additionally, we observe significant variations in crustal thickness within the overlying North American crust, associated with the migration of the MTJ and slab window. Using this new image of the crust-upper mantle structure in northern California we find that much of the on-shore and near-shore seismic activity in the region (including most of the larger events) occur within the overlying North American crust near or above the slab interface, rather than within the subducting slab. Additionally, this imagery indicates significant variations in crustal structure of the overlying North American crust associated with transitory basin formation and inversion associated with the migration of the MTJ.

Key Words
tomography, triple junction, plate tectonics

Citation
Furlong, K., Herman, M., & McKenzie, K. (2026, 08). Seismic Imaging of the Mendocino Triple Junction: Defining the Geodynamics of an Evolving Plate Boundary Transition. Poster Presentation at 2026 SCEC Annual Meeting.

Related Projects & Working Groups
Earthquake Geology