The Effect of Fore-Arc Deformation on Shallow Earthquake Rupture Behavior in the Cascadia Subduction Zone

Within the fore-arc of the Cascadia Subduction Zone, there are significant along-strike differences in the orientation of splay faults, sediment consolidation, and fault roughness. Here, we use dynamic rupture simulations of megathrust earthquakes on different realizations of a fault system that incorporate fore-arc properties representative of offshore Oregon and Washington to estimate how splay faults may behave in future megathrust earthquakes in Cascadia. While splay faults were activated in all of our simulations, splay orientation is a primary control on slip amplitude. Seaward vergent faults accommodate significant amounts of slip resulting in large seafloor uplift and significantly larger tsunami amplitudes. For example, our median tsunami heights including splay faults are about a factor of two larger than those that did not include splay fault deformation. We suggest that there is an urgent need to revisit existing approaches to tsunami hazard assessment in Cascadia to include the influence of splay faults. Plain Language Summary The Cascadia subduction zone has hosted many great earthquakes in the past. Due to the potential of generating great earthquakes, this zone poses a huge earthquake and tsunami hazard. The main fault (or Cascadia megathrust) responsible for generating great earthquakes is about similar to 1,000-km long and is inferred to have variation in the geometrical and mechanical properties. One such difference is the difference between properties of the "forearc" region. The "forearc" is the shallow subsurface region on the landward side adjacent to the Cascadia megathrust. The orientation of the secondary faults (i.e., splays) is different in the forearc for the offshore Washington and Oregon region. Similarly, the consolidation of sediments within the forearc is also different for the Washington and Oregon region. In this study, we performed earthquake and tsunami simulation to understand how the difference in these forearc properties affects the tsunami hazard of the region. We found that the tsunami hazard in the Washington region is higher as compared to the Oregon region due to higher magnitude slip on the splay and megathrust. We find that this difference is mainly due to the difference in the orientation of splay faults. Key Points Dynamic earthquake rupture simulations of the Cascadia Subduction Zone (CSZ) offshore Washington and Oregon region to investigate updip rupture behavior Larger tsunami amplitudes observed for the Oregon region suggest a large tsunami hazard in central and southern Cascadia Different orientations of the splays within the CSZ forearc result in differences in the amount of slip and tsunami heights