Variations in Subduction Bending-Related Normal Faulting of the Incoming Juan de Fuca Plate Along the Cascadia Margin

Cascadia Subduction Zone (CSZ) is a convergent boundary between the oceanic Juan de Fuca (JdF) plate system and the continental North American plate and has ruptured in at least 19 great megathrust earthquakes over the past 10 kyr. The controlled source seismic data acquired along the margin dating back to the 1980s show along-strike variations in faulting and hydration of the incoming plate; however, the available coverage is limited. More extensive and deeper penetrating subduction bend faults and associated plate hydration within the crust are inferred in the offshore central Oregon region than offshore central Washington. How faulting patterns develop across the incoming oceanic JdF plate system in response to oblique subduction resistance and other stresses and how these faults may impact hydration of the plate prior to subduction are not well understood. During June-July 2021, we ran the Cascadia Seismic Imaging Experiment (CASIE21) and acquired ~5347-line km of new marine multichannel seismic reflection (MCS) data covering much of the CSZ, from Northern Vancouver Island to Southern Oregon. Data processing was done by ION Geophysical to pre-stack depth migrated reflection images. To aid the interpretation of the reflection images, we use the corresponding velocity models and analyze a suite of seismic attributes such as spectral decomposition trace, instantaneous phase, envelope, and relative acoustic impedance. We then interpret the eighteen margin-normal MCS profiles with the goal to: (1) Characterize the distribution, geometry, and penetration depths of the bend-related normal faulting within the sediments, oceanic crust, and uppermost mantle; (2) Analyze the regional differences in faulting patterns of the incoming oceanic plate from the mapped normal faulting; (3) Investigate possible causes for the inferred different bend faulting patterns. For (3), we focus on (a) obliquity of convergence and hence orientation of pre-existing fault fabric to trench, (b) age of the plate at the deformation front, which varies from 4-10 Ma, (c) variations in the dip of the subducting plate, and (d) proximity to accreted crustal blocks in the overriding plate.