New map outlines landslide risks in western Oregon

EarthquakesNew map outlines landslide risks in western Oregon
Published 2 March 2015

Landslides are already a serious geologic hazard for western Oregon. During an earthquake, however, lateral ground forces can be as high as half the force of gravity. The Coast Range is of special concern because it will be the closest part of the state to the actual subduction zone earthquake, and will experience the greatest shaking and ground movement. New landslide maps have been developed that will help the Oregon Department of Transportation determine which coastal roads and bridges in Oregon are most likely to be usable following a major subduction zone earthquake that is expected in the future of the Pacific Northwest.

New landslide maps have been developed that will help the Oregon Department of Transportation determine which coastal roads and bridges in Oregon are most likely to be usable following a major subduction zone earthquake that is expected in the future of the Pacific Northwest.
The maps were created by Oregon State University and the Oregon Department of Geology and Mineral Industries, or DOGAMI, as part of a research project for ODOT. They outline the landslide risks following a large earthquake on the Cascadia Subduction Zone. The mapping is part of ongoing ODOT efforts to preserve the critical transportation routes that will facilitate response and recovery.
“Landslides are a natural part of both the Oregon Coast Range and Cascade Range, but it is expected there will be a significant number of them that are seismically induced from a major earthquake,” said Michael Olsen, an assistant professor in the OSU School of Civil and Construction Engineering. “A massive earthquake can put extraordinary additional strain on unstable slopes that already are prone to landslides.”
An OSU release reports that landslides are already a serious geologic hazard for western Oregon. During an earthquake, however, lateral ground forces can be as high as half the force of gravity. The Coast Range is of special concern, officials say, because it will be the closest part of the state to the actual subduction zone earthquake, and will experience the greatest shaking and ground movement. The research identified some of the most vulnerable landslide areas in Oregon as parts of the Coast Range between Tillamook and Astoria, and from Cape Blanco south to the California border — in each case, from the coast to about thirty miles inland.
“Major landslides have been identified by DOGAMI throughout western Oregon using high-resolution lidar mapping,” Olsen said. “Some experts believe that a number of these landslides date back to the last subduction zone earthquake in Oregon, in 1700. Coast Range slopes that are filled with weak layers of sedimentary rock are particularly vulnerable, and many areas are already on the verge of failure.”

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