The tsunami that devastated coastal areas of South Asia could occur along the coast of Washington and Oregon. Scientists and emergency managers say it's only a matter of time.
"This is a wake-up call for us," said George O. Crawford, earthquake program coordinator for the Washington Emergency Management Division. "This could be our tsunami."
The last tsunami to brush the Northwest came on March 27, 1964.
It began in an area off the southern shore of Alaska, where two tectonic plates had been grinding against one another for centuries. At 5:36 p.m., the Pacific plate jerked 30 feet beneath the North American plate. The 9.2-magnitude temblor struck with the force of about 63,000 Hiroshima-size atomic bombs, according to the U.S. Geological Survey, and sent waves of water surging along the coasts of Washington and Oregon.
A total of 131 people died, with most killed by tsunami waves that struck as far south as Crescent City, Calif.
In the Pacific Northwest, scientists say a new system of offshore buoys and seafloor sensors should provide two to three hours of warning in the case of a tsunami as distant as the one generated off Alaska's shore. Real-time data from the new buoys are transmitted to 24-hour tsunami warning centers, one in Alaska and one in Hawaii, operated by the National Oceanic and Atmospheric Administration.
"Two years ago, I would have said we're in big trouble," Crawford said. "Now, the West Coast is pretty well covered."
Seven communities in Washington and Oregon have been declared "tsunami ready" by the National Weather Service, but scientists and emergency managers say more could be done to raise public awareness and protect critical structures along the coast. Blue tsunami evacuation signs direct residents and visitors toward higher ground, but officials said they may have as little as 15 minutes to get the word out if a similar subduction zone quake occurs off the Northwest coast instead of near Alaska.
"If you're visiting the coast and you feel the ground shake, head to high ground," Crawford said.
Indeed, the least of our problems may be the severe damage certain to occur along the sparsely populated Washington and Oregon coastlines
"For us here in the Portland-Vancouver area, or the Willamette Valley in general, shaking would be our biggest effect," said Evelyn Roeloffs, a geophysicist with the USGS in Vancouver. "It would be quite significant, and it will continue for several minutes. Structures that can handle those forces for 10 seconds or so are going to have a hard time standing up if they endure it for several minutes."
Scientists say a tectonic subduction zone runs 50 miles off the Washington and Oregon coasts, raising the potential for tremendous damage quickly.
The Cascadia subduction zone, where the Juan de Fuca plate plunges beneath the North American plate, last jerked violently on Jan. 26, 1700. The quake generated tsunami waves that swamped coastal Japan. Strain between the two plates will continue to build, until the stress is unleashed in another powerful jerk.
Scientists say the interval between such events ranges between 200 and 1,000 years.
Sunday's quake in the Indian Ocean bore an "uncanny resemblance" to the temblor that struck the Northwest 304 years ago, said Brian Atwater, a USGS geologist based at the University of Washington.
"It's hard to predict what would happen in Portland and Vancouver in a magnitude-9 Cascadia event," said Atwater. "How the bridges would fare, high rises, these are the big open questions about Cascadia earthquakes."
Harry Yeh, an Oregon State University engineering professor, said many coastal communities have well-established evacuation routes, but those that don't could benefit from "tsunami shelters." He said he's visited five or six tsunami-affected areas, and found that reinforced concrete tends to withstand the force of surging water. Public buildings such as schools or libraries, built tall enough, could be strengthened and designated as shelter in case of a tsunami.
"Then perhaps people can evacuated vertically," he said, adding: "The only problem is the building has to be high enough. Otherwise, people drown."