Seasonal and inter-seasonal soil moisture controls on coseismic shallow landsliding near the Cascadia Subduction Zone

High levels of soil moisture tied to antecedent rainfall may predispose hillslopes to failure during earthquakes, but due to the infrequency of large earthquakes in a single region and lack of associated landslide inventories with well-characterized soil moisture data, characterizing the influence of soil moisture on coseismic shallow landsliding remains challenging. We used RegionGrow3D – a three-dimensional slope stability model that simulates landslide initiation on a regional scale – to simulate coseismic shallow landslides in a region of the Oregon Coast Range, USA, near the Cascadia Subduction Zone. Using this framework, we quantified the influence of seasonally dependent soil moisture, seismic acceleration, and soil shear strength uncertainty on landslide density, landslide scaling relationships, and spatial distributions of yield accelerations. We also demonstrated that, in some cases, seasonality and soil moisture may be a stronger control on coseismic landsliding than peak ground accelerations.