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Emerald Group Publishing Limited
Copyright © 2011, Emerald Group Publishing Limited
Old Pacific NW dams vulnerable to likely earthquakes
Article Type: News items From: Disaster Prevention and Management, Volume 20, Issue 1
Most dams in the Pacific Northwest are more than 20 years old – built before the explosion in geologic information about the region’s earthquake potential, according to several experts. Newly developed data on the Pacific Northwest show that the region has historically seen magnitude 9.0 quakes, and most of these dams were not built to withstand so powerful a shaking, according to Ivan Wong, the manager of the Seismic Hazards Group.
Data indicate that there have been about 20 large quakes – magnitude 9.0 or larger – affecting the region in the last 10,000 years. They do not appear to be evenly spaced, but come in clusters with long periods of dormancy. The chief geological threat in the region is the sub-duction zone where the Juan de Fuca continental plate is sliding beneath the North American plate in the Cascadia Subduction Zone.
US Geological Survey geologist David Atwater told the Association of Dam Safety Officials on September 20, “We have a history of magnitude 9.0 quakes in the Pacific Northwest,” although the last quake that large occurred around 1700. The area at risk from a quake in this region includes Seattle and Portland, and all of the coastal area of Washington, Oregon, and Northern California.
Much more information about historical seismic activity in the Northwest has been discovered over the last two decades. Scientists think the region is on a par with the better known earthquake hazards in California. Atwater said, “Most scientists now consider Cascadia to be a region of moderate to high hazards, similar to California. This obviously poses a challenge to dam owners and regulators whose responsibility it is ensure the safety of dams.” There was some hopeful data resulting from the recent Mw 9.2 quake in Chile. The quake struck over a large area – an equivalent length along the coast of the distance between Los Angeles and San Francisco. But retaining walls and dams performed well, with only one earthen dam showing minor distress, according to David Frost, an engineer at Georgia Tech.
The significance of the Chile results to the USA is that prior to the quake, Chile had adopted seismic codes similar to those in use in the USA. “Part of the reason it’s relevant to the United States is that it tested the codes we use,” Frost said.
One tailings dam at a closed-down gold mine did fail. Up to 100,000 cubic meters of tailings flowed about a half a kilometer (0.3 miles) and killed four people. Soil at the dam liquefied at the site during the main quake and aftershocks
(Natural Hazards Observer, November 2010)