Automated 3D dynamic rupture simulations for rapid characterization of large earthquakes: Application to the December 5, Mw 7.0 Off-shore Cape Mendocino earthquake

The December 5, 2024, M_w 7.0 Off-shore Cape Mendocino earthquake, the largest in California since the 2019 Ridgecrest events, struck approximately 70 km southwest of Ferndale, near the Mendocino Triple Junction (MTJ) where the Pacific, North American, and Juan de Fuca/Gorda plates converge. The MTJ is California's historically most seismically active region, which has experienced multiple up to M_w 7 earthquakes over the past few decades. Understanding the dynamics of this earthquake is essential to better understand regional seismicity, structural and stress heterogeneity, and the resulting stress redistribution onto two adjacent high-hazard fault systems, the Cascadia subduction zone to the North and the San Andreas transform fault system to the South. 

Rapidly characterizing large earthquakes is vital for effective disaster response and seismic and tsunami risk mitigation. Current assessments often rely on rapidly generated static or kinematic finite-fault models derived from geodetic data, teleseismic body waves, CMT solutions, scaling relationships, and regional waveforms (e.g., Hayes, 2017; Goldberg et al., 2022). Such models can also inform 3D dynamic rupture simulations, providing a physics-based perspective on earthquake behavior (e.g., Jia et al., 2023; Hayek et al., 2024).

In this presentation, we apply a new automated workflow to rapidly characterize the rupture dynamics of the recent M_w 7.0 Off-shore Cape Mendocino earthquake. We compare several reference finite-fault models,  including those from the USGS, SLPINEAR/GeoAzur, and a new static geodetic inversion, to automatically constrain 3D dynamic rupture simulations.  An ensemble of dynamic rupture models is explored, informed by the stress change of each finite-fault model, respectively. Preferred dynamic rupture models are automatically selected based on matching regional waveforms and moment rate release. This simple workflow can systematically assess the dynamic viability of kinematic slip models. 

While the static geodetic inversion reveals a main ruptured asperity that broke a strongly coupled section of the fault and rupture ceasing at a previously identified creeping section of the fault, the associated dynamic rupture models cannot explain the complex rupture dynamics imprinting, e.g., on moment rate release. Instead, we find that a higher degree of smaller-scale initial stress complexity, such as resulting from the SLIPNEAR model,  is required to explain observations. The resulting asymmetric rupture dynamics present challenges to rapid data-driven analyses and have significant implications for understanding future earthquakes in the Mendocino Triple Junction region.