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

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

Continued on Next Page >>


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