Landscapes Live: Exploring the timing, triggering and spatial distribution of landslides along the Cascadia Subduction Zone
The last decade has provided unexpected lessons in the enormous risks from great subduction earthquakes: Sumatra 2004, Chile 2010, and Japan 2011 were each devastating, resulting in surprising impacts distinct from shallow seismic events. Similar large-magnitude earthquakes are known to occur on the Cascadia subduction zone, with the potential of rupturing the entire 1100 km length of the Pacific Northwest plate boundary. Because a magnitude 9 (M9) subduction earthquake is well known to have occurred just over 300 years ago, evidence of coseismic landslides triggered by this event might still be present in the landscapes of the Washington and Oregon Coasts. Indeed, the coastal Pacific Northwest USA hosts thousands of deep-seated landslides, yet little is known about their timing or what triggered them. In this talk, I will show our new map of 9,938 deep-seated bedrock landslides in the Oregon Coast Range and explain how we used surface roughness dating to estimate that past earthquakes triggered less than half of the landslides in the last 1,000 years. Comparison of our inventory with other geophysical factors suggests that landslide frequency increases with mean annual precipitation but not with modeled peak ground acceleration or proximity to the megathrust. Our results agree with findings from other recent subduction-zone earthquakes where few deep-seated landslides were mapped, and suggest that despite its proximity to the megathrust, most deep-seated landslides in the Oregon Coast Range were triggered by rainfall.
Speaker: Alison Duvall (University of Washington)