Using numerical modelling to investigate the driving forces of permanent forearc deformation in northern Cascadia

We use boundary element method modelling to investigate whether subduction zone coupling drives permanent forearc deformation in the northern Cascadia subduction zone. Recent work in this region shows that several active crustal faults accommodate permanent strain north of the Olympic Peninsula in Washington State, USA and British Columbia, Canada. These faults are similar in that they strike west-northwest, have oblique right-lateral slip senses, and have low slip rates (<1 mm/yr). Paleoseismic studies show that despite the region’s low permanent strain rates, these faults have produced large (~M 7) earthquakes. Therefore understanding how and why these structures accommodate permanent deformation is crucial to assessing regional seismic hazard. Previous work has hypothesized this type of permanent forearc deformation may be driven by stress resulting from interseismic subduction zone coupling. To test this hypothesis, we used a 3D boundary element method model to determine whether coupling-driven forearc deformation can account for the observed right-lateral fault slip on one of the recently studied structures, the Leech River--Devils Mountain fault. Our model predicts left-lateral slip on this fault if strain results from subduction zone coupling alone, inconsistent with the observed kinematics. Additionally, if we use our model to mimic strain partitioning, where only strain resulting from the strike-slip component of subduction zone coupling is accommodated in the forearc, the predicted fault slip is also inconsistent with observations of fault kinematics. These simplified models represent a first-order test that contradicts the hypothesis that subduction zone coupling is the primary driver of permanent forearc deformation in northern Cascadia.