DEVELOPING A REGIONAL LANDSLIDE CHRONOLOGY NEAR OSO, WASHINGTON (USA) USING TOPOGRAPHIC ROUGHNESS MEASURED FROM LIDAR IMAGERY (Invited Presentation)

In landslide prone regions worldwide, accurate risk assessments are hampered by a dearth of historical landslide chronologies. Advancing from hazard recognition to risk assessment requires knowledge of how frequently landslide events occur, but despite recent advances in landslide mapping due to the availability of lidar imagery, estimating the timing of slope failures remains a challenge. Geomorphologists have long recognized that the topographic roughness of landslide deposits decreases over time due to soil production and transport. Here, we show that this observation can be used as a quantitative tool for landslide age estimation, when combined with even a small set of radiocarbon dates. Using high resolution (1m) lidar elevation data, we measure the surface roughness for hundreds of deep seated landslides in glacial sediment lining the valley walls of the North Fork Stillaguamish River near Oso, Washington. By plotting surface roughness values against 17 radiocarbon dates that constrain landslide timing, a predictable exponential decay relationship can be drawn and used to estimate the age of every landslide in the valley. With this complete landslide chronology we can examine trends in landslide activity over time and space, and more accurately estimate recurrence interval. Application of this technique to bedrock landslides of the Oregon Coast Range is currently underway. We hope this effort will eventually reveal coseismic landslides related to Cascadia Subduction Zone earthquakes.