Redefining the 3-D Plate Boundary Structure of the Mendocino Triple Junction (MTJ)

Sarah Kummerfeldt, Kevin Furlong, & Kirsty A. Mckenzie

Submitted August 30, 2026, SCEC Contribution #15505, 2026 SCEC Annual Meeting Poster #TBD

This study focuses on the region near the Mendocino Triple Junction (MTJ), where the Gorda slab geometry is not well constrained in terms of its depth and configuration. As a result, earthquake studies have reached disparate conclusions for the tectonic setting of the seismic activity. Here we better define the Gorda slab geometry, which includes delineating the southern edge of the slab. In addition, we determine the tectonic setting of two moderate, recent regional earthquakes: the M6.2 earthquake near Petrolia, CA in 2021, and the M6.4 earthquake near Ferndale, CA in 2022. Using a high-resolution crustal/upper mantle tomography model (Furlong et. al., 2024), we define the Gorda slab position and geometry, and correlate seismic velocity with the geology in this region. In the vicinity of the MTJ, the North American crust is primarily made up of the Franciscan Complex, which has P-wave velocities that range from ~ 5.0 km/s to ~ 6.8 km/s, whereas the Gorda plate is expected to have velocities >7.0 km/s. By combining seismic tomography with the seismicity dataset, each hypocenter is correlated to the tomography and the seismic velocity at that location. This method allows us to link each earthquake to the crustal component and associated tectonic setting where it is occurring. This earthquake-tectonics correlation is aided by the rock units in this region having distinct velocity ranges. We have determined that the slab edge extends to the east from the eastern end of the Mendocino Fault at a 100-degree azimuth, more easterly than previously interpreted. The 2021 Petrolia event occurred within the slab along or near its southern edge, while the 2022 Ferndale event occurred above the slab within the crust of the North American plate.  

Key Words
Tectonics, Seismicity, Mendocino Triple Junction, Tomography

Citation
Kummerfeldt, S., Furlong, K., & Mckenzie, K. A. (2026, 08). Redefining the 3-D Plate Boundary Structure of the Mendocino Triple Junction (MTJ). Poster Presentation at 2026 SCEC Annual Meeting.

Related Projects & Working Groups
Plate Boundary System (PBS)