Redefining the 3-D plate boundary structure of the Mendocino Triple Junction (MTJ) with earthquakes and seismic tomography

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 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 distinct velocity ranges for rock units in this region. Our results show 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 Gorda slab along or near its southern edge, while the 2022 Ferndale event occurred above the slab within the crust of the North American plate.