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Is Tsunami Threat to U.S. West Coast Bigger Than Predicted?

John Roach
for National Geographic News
December 21, 2005
 
A major earthquake off the U.S. West Coast could produce tsunami waves
that send water surging up the coast to much greater heights than
previously anticipated, according to scientists.

The finding stems from information gathered during a research expedition to the seafloor near the epicenter of the earthquake that caused the December 26, 2004, Indian Ocean tsunami.

The team's observations differed from what they expected to find based on computer models run for this event, sonar observations made by a British ship, and previous observations of earthquakes and tsunamis.

For example, the researchers found scant evidence for fresh underwater landslides, which early mapping suggested were responsible for the giant waves.

Instead the team found a 12-mile-long (20-kilometer-long) feature nicknamed "The Ditch" where the seafloor near the epicenter was vertically displaced by up to 39 feet (12 meters)—more than twice as high as expected.

Stephan Grilli, an ocean engineer at the University of Rhode Island in Narragansett, was one of the researchers involved in the study.

Grilli applied the new information to a computer model of a fault off the Oregon coast. He then measured the predicted wave run-ups, the rush of water up a slope or structure that is associated with tsunamis.

He found that a major earthquake along the Oregon fault could produce wave run-ups of up to 98 feet (30 meters) in some locations.

Previous modeling suggested wave run-ups of up to 50 feet (15 meters) were possible from an earthquake of magnitude 9.1 on the fault.

"This is a point of concern," Grilli said. "We probably need to take another look at the scenarios we have used, [as] we have learned new things about large earthquakes and tsunamis by studying the Indian Ocean."

Grilli presented his findings earlier this month at the annual meeting of the American Geophysical Union in San Francisco, California.

Results Questioned

Timothy Walsh is a geologist with the Washington State Department of Natural Resources who has worked on models predicting the height of tsunami-triggered run-ups along the West Coast.

He says the new information from the Indian Ocean is important and will eventually improve models used to study tsunamis in the Pacific Northwest.

Walsh notes, however, that Grilli's current application of the data to the Pacific Northwest is oversimplified.

"It was for illustrative purposes that they did this simple model," he said. "I've been in touch with Professor Grilli and looked at what he did. It was very simple and in several areas was demonstrably wrong."

For example, Walsh says, Grilli's model misplaces the location of where the two tectonic plates that form the fault come together in much of Oregon and Washington. It also misrepresents how they interact.

This resulted in much more offshore vertical displacement of ocean crust than will actually occur during a tsunami-producing quake, Walsh says. Greater displacement of ocean crust generates larger waves, he explains.

In addition, Walsh says, the resolution on Grilli's model fails to pick up the nuances of local geographic topography. As a result the model is unable to detect how local features interact with the killer waves, potentially reducing their impact.

The models that Walsh and his colleagues use to evaluate earthquake and tsunami hazards in the Pacific Northwest are at a much finer scale, giving them confidence in their results.

"We're pretty comfortable that the models we've been using are appropriate to use," Walsh said. "But we'll still do refinements as information becomes available."

For example, Grilli's direct observations will help Walsh and colleagues resolve their uncertainties in estimating seafloor deformation.

Frank Gonzalez leads the tsunami research program for the National Oceanic and Atmospheric Administration in Seattle, Washington. He said Grilli's modeling results are "very preliminary" and will likely change as the resolution is increased.

"The devil's in the details," he said. Nevertheless, he added, "nature is pretty brutal, and someday we'll have on [the West Coast] an event quite similar to the one on Sumatra."

Tsunami Preparedness

Grilli agrees that the tsunami source he used in his new computer model of the Pacific Northwest fault is preliminary and was done purely for illustrative purposes.

He is now collaborating with local earthquake and tsunami researchers in the Pacific Northwest, including Walsh, to improve it.

But, he says, the physics of the model itself are more complete and thus could still produce more accurate results, given the same data, than the models currently used by his colleagues.

He hopes the collaboration will result in a more "realistic" prediction for the earthquake and tsunami hazard in the region. More accurate models, he adds, will allow agencies to be better prepared when disaster strikes.

"We have the tools and we have the knowledge to go at this problem and improve the state of the art and be better prepared."

Asked if he thought the Pacific Northwest is prepared for a tsunami, regardless of the wave run-up height, Walsh said, "No, nobody is."

"We're better than we were ten years ago, but we're still working on it."

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