12/07/2005

Associated Press

Washington state's evacuation plans for a tsunami along the coast assume a worst-case wave 30 feet high. But a new analysis based on the Indian Ocean tsunami says a quake off the Northwest coast could generate waves two to three times as big — 65 to 98 feet high.

The new analysis, based on last year's deadly Indian Ocean tsunami, is based on a computer model at the University of Rhode Island.

The results are preliminary, but still troubling, said Stephan Grilli, chairman of the university's Department of Ocean Engineering.

"It raises a flag," he told The Seattle Times for a report Wednesday. "I think revising the hazard assessments would be a wise thing to do."

Tim Walsh, geologic-hazards manager for the state Department of Natural Resources, is skeptical.

"Sumatra is causing some people to think we could have something worse than what we're planning for," Walsh said. "I don't think so, but I'm willing to listen to the people making these arguments."

The 600-mile-line Cascadia subduction zone, a geologic fault off the Northwest coast, has generated tsunamis in the past. But Walsh says when it slips, the energy is directed sideways and not up — so it probably would not drive waves comparable to the 80-footers that slammed Sumatra a year ago.

State scientists and emergency managers will review the new computer-model results, however.

Vasily Titov, a tsunami modeler for the National Oceanic and Atmospheric Administration in Seattle, said his computer simulations suggest the possibility of waves up to 65 feet in some stretches of the Washington and Oregon coasts.

He says he initially thought the numbers were ridiculously high. Then he went to Banda Aceh, on Sumatra's north coast.

"If such a huge event could happen there, there is no doubt it could happen here," he said.

Grilli and colleagues went to the stricken region a few months after the tsunami. They used seismic instruments and underwater cameras to gauge ground movement during the magnitude-9-plus earthquake that drove the waves.

In some places, the uplift was nearly 40 feet, he said.

The Discovery Channel and the British Broadcasting Corp. (BBC) helped pay for the trip. "America's Tsunami: Are We Next?" a Discovery Channel program based on the recent research, will air Dec. 18.

Grilli also will discuss his work this week at the American Geophysical Union's annual meeting in San Francisco.

He used his findings in Sumatra and a state-of-the-art computer model to simulate a magnitude-9.2 earthquake on the Cascadia subduction zone. In his scenario, the entire fault ruptured, with displacements of about 60 feet.

The parameters seem reasonable, said University of Washington geologist Brian Atwater, the first to discover that a Cascadia quake 300 years ago sent a tsunami toward the Northwest coast.

It's hard to tell how big those waves were, he said. Japanese records say the tsunami was as much as 15 feet high after it crossed the Pacific.

The state's current worst-case scenario of a 30-foot wave is based on older computer models and a simplified view of how the fault might rupture in a major earthquake, Walsh said.

The Sumatra quake showed that the amount of uplift can vary greatly along the length of the fault. But experts really don't know how to predict which areas might slip the most, the key to predicting wave action, Walsh said.

It is almost impossible to prepare for 90-foot waves hitting the coast within 30 minutes of a major quake, Walsh said. Low-lying areas such as most of the Long Beach Peninsula would likely be under water before residents could reach higher ground.

Even with current wave-height forecasts, some county planners have been struggling to come up with high-ground gathering places. Grays Harbor County has proposed 19 gathering spots, but only seven are 90 feet or more above sea level. Seven are at elevations less than 30 feet.

The city of Ocean Shores has an evacuation gathering site at a 52-foot elevation. But it is unclear how people could reach that site before a tsunami hit, with the main road clogged with cars.

Grilli will discuss his work this week at the American Geophysical Union's annual meeting in San Francisco.

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On the 'Net:

Department of Natural Resources tsunami maps, http://www.dnr.wa.gov/geology/hazards/tsunami/evac

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Information from: The Seattle Times, http://www.seattletimes.com