CASCADIA SUBDUCTION ZONE LANDSLIDE HAZARD FORECASTING

The consequences of a great (M9.0) earthquake along the Cascadia Subduction Zone (CSZ) are highly uncertain due to the lack of available data on consequences of the most recent 1700 CSZ earthquake, and the complexities involved in forecasting hazard for such a large magnitude earthquake. Of particular concern are coseismic landslides triggered by this earthquake that may affect a vast area of the Pacific Northwest, causing direct losses through impacts to the built environment as well as isolating communities from first responders by damaging or blocking roadways. Numerical modeling and oral histories suggest large landslides may have been triggered from southern Oregon to Vancouver Island, B.C during the 1700 CSZ earthquake, and recent megathrust earthquakes (e.g., Tohoku, Peru, Chile) have triggered thousands of landslides in a wide range of failure modes. These different modes of landslides range from shallow disrupted soil slides to flow-slides and lateral spreads are associated with different failure mechanics and degrees of consequences to the built environment and natural landscape. We first develop a suite of physics-based models reflecting the individual nature of common landslide failure modes. This suite of models is then used to predict coseismic landslide hazard for the Pacific Northwest for M9.0 earthquake scenarios using physics-based synthetic ground motions that were developed using a 3-D seismic velocity model of the Cascadia subduction zone. Composite hazard maps, depicting the location and landslide failure mode associated with each hazard area, are shown for the Pacific Northwest and Seattle, Washington.