Widespread on-fault geologic evidence for variable mode coseismic slip to the trench in Cascadia

Investigating the geologic record of near-trench coseismic strain release is critical for estimating the slip distribution and tsunamigenic potential of future large subduction zone earthquakes. However, the style and extent of upper plate coseismic deformation offshore in subduction zones is poorly constrained. To address this knowledge gap, we integrate shipborne multichannel seismic and multibeam data with targeted autonomous underwater vehicle (AUV) bathymetry and chirp data to resolve the updip limit and mode of coseismic slip along the frontal thrust fault zone in central and southern Cascadia. Detailed fault mapping and characterization reveals four distinct styles of Holocene deformation along the frontal thrust fault zone, including fault propagation folding, imbricate thrusting, breakthrough thrusting, and frontal erosion through widespread mass wasting. Joint interpretation with crustal-scale seismic data shows that these variations in near-surface deformation style are clearly linked to changes in megathrust geometry and accretionary mode at depth. Additionally, AUV data provide abundant on-fault geologic evidence for discrete and episodic near-surface deformation, including primary fault and fold scarps, bending moment fault scarps, punctuated growth strata, and discrete single event fault offset, that we consider to be predominantly coseismic. The widespread distribution of Holocene active, likely coseismic, frontal thrust deformation presented in this study highlights the potential for variable mode coseismic slip to the trench in Cascadia. These results provide an unprecedented view into the shallow megathrust structure and heterogeneity in Cascadia that is critical for estimating earthquake and tsunami hazard impacts locally and for understanding shallow megathrust slip behavior globally.