by Helen Caswell
Salem residents concerned about the coming Cascadia subduction earthquake can cross one potential catastrophe off their lists, says the US Army Corps of Engineers.
And that is t he disaster of the Detroit Dam falling to pieces and its water engulfing the city.
It’s a topic local people wonder about. “I’ve been asked repeatedly about Detroit Dam failure in a Cascadia [earthquake] since I began this job,” says Ed Flick, Marion County Emergency Manager. “The consequences would be catastrophic, but the likelihood is extremely remote.”
Completed in 1953, the Detroit Dam is only 45 miles east of Salem and was built before Oregonians knew the risk of earthquakes in the Pacific Northwest . It wasn’t until the 1970s that people began to understand the state had numerous faults both off shore and onshore. In particular, geologists learned the region is subject to massive Cascadia Subduction Zone quakes which occur about once every 300 years.
It’s now been 316 years since the last Cascadia , but Matt Craig, Dam Safety Program Manager for the U.S. Army Corps of Engineers, says that though the Detroit Dam was not constructed with seismic concerns in mind, the corps does not anticipate a failure.
“We do risk assessments on an ongoing basis,” he says, “in particular of what might happen in a 9.0.”
A magnitude-9.0 earthquake is the most intense possible quake, and only occur at subduction zones like the one off the coast of Oregon. “We look at whether an event like this could lead to dam failure,” Craig continues, “and what is the likelihood of some kind of release of water.”
Craig oversees the Corp’s “Portland District;” 20 dams that run roughly north to south from the Columbia River down to just above the California border. Thirteen are in the Willamette Valley basin. All twenty have a safety inspection every five years and a seismic study every 15 years.
Detroit Dam is built out of concrete “monoliths” Craig says , enormous slabs held together by joints. “They were purposely given joints so they could respond to temperature changes.” But although the Corps “would expect some movement , that won’t cause enough damage to release water. We don’t believe the blocks will separate at the joints.”
The dangers from earthquakes in Oregon are real. Salem Weekly encourages all readers to get information from the Oregon Office of Emergency Management or the Red Cross. Or, visit the City of Salem’s Emergency Management page and sign up for CERT disaster preparedness training.
In fact, Craig says, dams aren’t as susceptible to earthquakes as people might think; of dam failures around the world where every level of construction expertise is employed, only 1% can be attributed to earthquakes. He points to the 2011 magnitude-9.0 earthquake in Fukushima, Japan, that caused major devastation.
“It was similar to what we’d see in a Cascadia Subduction Zone event,” Craig says. But of over 250 dams in the Fukushima impact zone, only one small earth dam failed.
Of greater concern to emergency managers is the possible collapse of infrastructure like roads and bridges, broken water and gas lines and downed power lines.
Flick, whose goal is to have Marion County “the most prepared region in the state,” says that he and other managers receive regular information and training on dam breakage risks, and though the science doesn’t suggest a dam that impacts the county will fail in an earthquake, “that’s not to say we don’t prepare for it and review our plans annually. But it’s not a high concern.”