The complex rupture dynamics of an oceanic transform fault: supershear rupture and deep slip during the 2024 Mw7.0 Cape Mendocino Earthquake

Thomas Ulrich , Yohai Magen, Alice-Agnes Gabriel

The December 5, 2024, Mw7.0 Cape Mendocino earthquake ruptured an oceanic transform fault within the tectonically complex Mendocino Triple Junction (MTJ), the most seismically active region of California and caused a soon-lifted tsunami evacuation alert. Its offshore location renders accurate analysis of source characteristics challenging. We integrate back-projection, geodetic and kinematic slip inversions, ensembles of hundreds of 3D dynamic rupture simulations, Coulomb stress modeling and regional velocity models to understand the event's rupture dynamics and implications. A preferred dynamic rupture scenario that matches seismic and geodetic observations is complex and asymmetric, despite the simple fault geometry, its extent limited by the Mw7.0 1994 earthquake and creeping fault portion. Driven by prestress heterogeneity and fault weakness, we find localized supershear rupture, and delayed deep slip of eastern fault portions where seismic and aseismic slip may coexist. The modest dynamic and static stress changes onto the adjacent Cascadia and San Andreas fault systems offer insight into possible future stress transfer pathways in the MTJ region. Our findings have important implications for the expected earthquake complexity at oceanic transform faults worldwide, and emphasize the need for improved offshore observations to support physics-based hazard assessment for offshore fault systems, including the MTJ.