Subduction intraslab-interface fault interactions in the 2022 Mw 6.4 Ferndale, California earthquake sequence

Abstract

The Mendocino triple junction, the intersection of the Pacific, North American, and Gorda plates, activates a collection of disparate faults that reconcile Cascadia subduction with San Andreas transform motion. The December 20, 2022, Mw 6.4 Ferndale, California earthquake occurred within this complex zone as strike-slip faulting within the subducting Gorda slab. Here, we analyze the seismic and geodetic signatures of the mainshock and aftershock sequence to illuminate its role within complex tectonic surroundings. We find aftershocks on varied fault structures within the uppermost Gorda slab, yet seismicity on the subduction interface itself was notably absent. Nevertheless, we identify small but coherent postseismic deformation that is well modeled by aseismic slip on this interface, likely triggered by stresses generated at the updip limit of coseismic rupture. This sequence demonstrates the potential for interactions between intra-slab earthquakes and slip on the subduction megathrust, highlighting the need to consider this and other subduction zones as coupled systems of interacting faults.