ASSESSING THE IMPACT OF COSEISMIC LANDSLIDES DURING THE AD 1700 CASCADIA SUBDUCTION ZONE MEGATHRUST EARTHQUAKE ALONG THE COASTLINE OF THE PACIFIC NORTHWEST (USA)

Coseismic landslides are one of the greatest secondary hazards of large earthquakes, and are often responsible for much of the total human and economic losses. In the Pacific Northwest, large earthquakes up to magnitude 9 have been shown to occur, on average, every ~300-600 years along the megathrust fault of the Cascadia Subduction Zone, which stretches 1100km from northern California to southern British Columbia. Predictive modeling of where landslides will be triggered during the next megathrust earthquake is imperative to reduce the level of risk faced by millions of people living in this region, but for such forward modeling to be credible, model results should be compared to known past events. Human records and tsunami deposits have pinpointed the timing of the last large megathrust earthquake to the year 1700, but the actual impact of this ground shaking on the landscape is poorly understood. This study seeks to quantify the coseismic landslide response to the 1700 earthquake by developing a chronology of landslide events across five swaths of the Oregon and Washington coastlines, where peak ground accelerations were likely to be greatest during the earthquake. Each of these swaths has high-resolution LiDAR bare-earth elevation data available, which allow for careful landslide deposit mapping and event delineation. By applying a recently developed surface roughness-radiocarbon age dating technique on a regional scale, we develop robust landslide chronologies for each study area swath. It is rarely possible to determine the triggering mechanism of any individual ancient landslide, which has made differentiating seismically induced landslides difficult in the past. However, complete regional landslide chronologies allow us to deduce patterns of landslide distribution in space and time, which can be compared to the timing of past seismic events, allowing for a more comprehensive and quantitative understanding of the past impact of coseismic landslides on the landscape of the Pacific Northwest.