WHAT CAN CASCADIA’S COASTAL LANDSLIDES REVEAL ABOUT COSEISMIC TRIGGERING AND THE PATTERN OF STRONG GROUND MOTION FROM MEGATHRUST EARTHQUAKES?

Deep-seated subaerial landslides are ubiquitous and well-mapped in western Cascadia, yet their triggering mechanisms are mostly unknown. Because this region is prone to intense rainfall and infrequent but large earthquakes along the Cascadia Subduction Zone megathrust, determining what triggers landslides is important for understanding landslide hazard and interpreting the onshore effects of past offshore earthquakes. Despite growing evidence that earthquakes triggered few of the deep-seated landslides in the mountainous Oregon Coast Range, prior work in the flatter uplifted marine terraces along the Oregon coastline reveals three individual landslides whose failure geometries could only be replicated by slope-stability models with the input of strong ground motions. In this study, we focus on wave-cut marine terraces along the coastlines of Oregon and Washington, where limited topographic relief and simple geometry offer a controlled environment to categorize landslides by potential trigger based on mapped plan-view morphology. Using the 3-D slope stability model Scoops3D on actual and idealized terrace geometries in concert with laboratory strength measurements, we show that increasing horizontal ground acceleration dramatically increases the size of potential failure surfaces. Landslides triggered by pore-water pressures alone are mostly confined to the terrace riser, whereas ground motions of ~0.4-0.6g can produce landslides that extend hundreds of meters further landward. By integrating these model results with an inventory of ~100 landslides along coastal marine terraces in Oregon, we assemble a dataset of landslides categorized by likely triggering mechanism. Although most landslides in the inventory have morphologies that do not preclude pore-pressure triggering, 10-20 landslides likely required strong ground motions to fail. Ongoing work will broaden the inventory to include landslides along the Washington coastline (where some landslides resemble the morphology of those likely triggered by earthquakes in Oregon), better constrain the shaking intensity required for slope failure, and interrogate patterns in estimated onshore shaking intensity near the Cascadia Subduction Zone margin.