EVIDENCE FOR EARTHQUAKE TRIGGERING OF LARGE LANDSLIDES IN COASTAL OREGON, USA

Large translational rockslides are common along the central Oregon coast but generally pose little hazard to human safety because of their slow movement (~cm/d). However, their reactivation and the formation of similar landslides during future subduction-zone earthquakes could have catastrophic results because many slides and slide-prone materials underlie tsunami evacuation and emergency-response routes. The most recent great subduction-zone earthquake off the Oregon coast occurred during Jan. 1700 with M 9, and such earthquakes have a recurrence interval of ca. 300-500 yrs. We studied three landslides typical of the central coast to evaluate possible causative factors and potential displacement accompanying a future earthquake. We characterized the landslides in the field and laboratory and monitored movement and hydrologic conditions for 3-8 yrs. Results from mapping and monitoring were used to estimate ages of two landslides. Slope-stability modeling was performed to evaluate factors of safety against initial landslide formation assuming pore-pressure increase or seismic shaking. We estimated coseismic displacement following the Newmark approach and using cyclic ring-shear tests; both methods use seismograms from the M 9.1, 2011 Tohoku Japan subduction-zone earthquake.

Observed rates and total landslide movement suggest initiation ages on the order of 350 yrs, which is close to the time since the last great earthquake (311 yrs). Stability analyses suggest that increased pore pressures did not initiate the slides (factor of safety >4), whereas moderate earthquake ground motion could have produced them (peak ground acceleration >0.6 g), and motion similar to that during the 2011 Japan earthquake would have produced them (factor of safety <0.7). Newmark analyses suggest minor coseismic displacement during landslide formation (<5 cm) but extreme displacement during reactivation of existing landslides (~10 m). Results from cyclic ring-shear tests are pending. Our initial findings suggest that many translational rockslides along the central Oregon coast were triggered by the 1700 M 9 earthquake and a similar future earthquake will trigger similar landslides that may hinder tsunami evacuation and block emergency-response routes.