Megathrust Earthquakes Amplify Coastal Wave Hazards in Cascadia Under Rising Sea Levels

Cesar Gavrie Lopez , Claire C Masteller 

Along the Cascadia Subduction Zone, destructive megathrust earthquakes represent a significant coastal hazard. These events generate abrupt coseismic subsidence, instantaneously increasing relative sea level. In the coming decades, global sea level rise will continue to shift baseline coastal conditions, potentially enhancing earthquake-driven hazards. Here, we model how sea level rise, wave climate, and the magnitude and timing of earthquake-driven subsidence interact to influence nearshore wave dynamics along the Cascadia margin. We find that coseismic subsidence amplifies shoreline wave power by shifting wave action landward despite reductions in wave breaking. This amplification is strongly controlled by shore platform slope, with negligible increases along steep shores and up to a million-fold increase in wave power for low-gradient sites. These changes highlight the potential for drastic increases in coastal erosion following future earthquakes. The timing and persistence of these hazards strongly depend on the sea level trajectory. Under a high-emissions scenario, accelerated sea level rise shifts the window of maximum earthquake-driven wave amplification earlier in the century, increasing the potential of post-event coastal recovery. Our results demonstrate that interactions between climate and solid Earth geohazards create transient, but potentially long-lived, shifts in coastal wave energy regimes that can shape future hazards and landscape evolution.