INVESTIGATION OF CASCADIA EARTHQUAKE TRIGGERED LANDSLIDES

Coseismic landslides will likely account for much of the impact associated with the next Cascadia Subduction Zone (CSZ) earthquake. Despite the dozens of Cascadia subduction zone events in the Holocene, the connection between megathrust earthquakes and landsliding has not been established, much less systematically tested. To examine the role of coseismic landsliding in the Pacific Northwest, we have identified 219 deep-seated landslide complexes in the Oregon Coast Range (OCR) that exhibit the signature of recent movement as seen with airborne lidar mapping as well as upstream stream sediment in-fill caused by landslide dam impoundment. The goal is to sample and date detrital organics and tree snags and correlate with the CSZ earthquake record, specifically focusing on morphologically young landslide deposits that were likely triggered by the 1700AD event.

To date, we have visited and performed reconnaissance at 11 sites. We sampled tree snags and organic detritus, and performed carbon 14 dating at 7 sites. We extracted wedges from tree snags for dendrochronology from 3 sites. We are also performing subsurface exploration with hand augers at selected sites. Finally, detailed site maps and a summary of the findings will be created in a final report. We will present a summary of our work to date.

In June 2017, we held a workshop to facilitate, coordinate, and expand collective efforts to characterize the complex coseismic landslide history and hazards in the Cascadia region. The workshop consisted of a day of presentations summarizing the current knowledge base, followed by brainstorming on topics that need future attention effort. This was followed by a field day visiting one of the 11 sites. The four primary future research topics developed by the workgroup are: 1) a better understanding of the earthquake effects on the landscape, 2) a compilation of current knowledge of landslides triggered by subduction zone events, 3) use of landslide data to assist in constraining earthquake recurrence intervals, and 4) use landslides to constrain ground motion.