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, meter-scale coseismic subsidence, instantaneously increasing relative sea level, inundation extent, and the reach of ocean waves. In the coming decades, eustatic sea level rise will continue to shift baseline coastal conditions, with the potential to change earthquake-driven coastal hazards. Here, we model how sea level rise, wave climate, and the magnitude and timing of coseismic subsidence interact to influence nearshore wave dynamics at 32 rocky coast sites spanning diverse morphologies along the Cascadia margin. We find that coseismic subsidence amplifies shoreline wave power by shifting wave action landward and reducing energy dissipation, despite an overall reduction in wave breaking. This amplification is strongly controlled by shore platform slope, with negligible increases along steep rocky coasts and multiple orders-of-magnitude increases for low-gradient platforms. These changes highlight the potential for increases in wave-driven coastal erosion and shoreline retreat following future earthquakes. The timing and persistence of these earthquake-driven hazards depend strongly on the sea level rise trajectory. Under a high-emissions scenario, accelerated sea level rise shifts the window of maximum earthquake-driven amplification toward the beginning of the century, increasing the potential of post-event recovery. Our results demonstrate that interactions between sea level rise and episodic coseismic subsidence create transient, but potentially long-lived, shifts in coastal wave energy regimes that can shape future coastal hazards and landscape evolution. This coupling between climate and solid-Earth processes reveals a previously unrecognized control on coastal change along tectonically active margins.