Inner forearc faults in northern Cascadia do not accommodate elastic strain driven by the megathrust seismic cycle

Abstract

We employ numerical models to explore the connection between subduction zone coupling or megathrust rupture and upper plate faults in the northern Cascadia forearc. Active forearc faults north of the Olympic Peninsula exhibit similar characteristics: west-northwest strike, oblique right-lateral slip senses, and low slip rates (<1 mm/yr), but a potential to generate large (M ~ 7) earthquakes. Previous hypotheses suggest that stress in the upper plate due to interseismic coupling or coseismic rupture along the subduction zone interface could drive permanent forearc strain. To test these hypotheses, we used a 3D boundary element method model to predict slip on the LRDM if interseismic coupling or coeseismic rupture cause deformation. Our model predicts reverse left-lateral slip if the strain results solely from subduction zone coupling, or normal right-lateral slip if these faults accommodate strain during a megathrust rupture. These results contradict the observed fault kinematics. Additionally, if we use our model to mimic strain partitioning, where only the strain from the strike-slip component of subduction zone coupling is accommodated in the forearc, our results are also inconsistent observed fault kinematics. These models challenge the hypothesis that subduction zone coupling or coseismic rupture are the primary driver of permanent forearc deformation in northern Cascadia.