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If big quake hits off coast, tsunami could be gigantic

David Perlman, Chronicle Science Editor

Monday, June 13, 2005

If a giant magnitude 9 earthquake strikes someday along the coast of the Pacific Northwest, or if, against all odds, an errant asteroid plunges into the ocean many miles off California, a monstrous tsunami could drown low-lying lands all up and down the continent's western edge -- and now a UC Santa Cruz scientist has calculated the sweep of such an event.

Spurred by the tragedy of December's great Sumatra quake and the hundreds of thousands of deaths claimed by the waves that swept across the Indian Ocean, geophysicist Steven Ward has estimated the heights that a similar quake-spawned tsunami would reach, running up along the shores from British Columbia as far south as the tip of Baja California.

"We need to know what the tsunami dangers are along any coastal area," Ward says, "and as our instruments and technology and modeling techniques improve, so we can refine our ability to forecast what might happen."

Using knowledge gleaned from evidence of a magnitude 9 quake in the Cascadia subduction zone some 300 years ago, the behavior of last December's Sumatra quake, careful scrutiny of detailed ocean bottom data all along the Pacific Coast and what he calls "the laws of water physics," Ward has created a hazard map that shows what may happen should another major quake hit the same area in the future. The Cascadia zone is a region where the eastern edge of a great undersea slab of the Earth's crust, called the Juan de Fuca Plate, is continually diving beneath the west edge of the North American Plate and thrusting the continental side of the crust upward.

To model the event's effects, Ward assumes that in a huge quake on the Cascadia subduction zone, the two crustal plates would abruptly slip apart vertically by at least 50 feet in three successive blocks from south to north, generating a 9.2 magnitude quake. Aside from enormous quake damage on land for hundreds of miles, Ward estimates the resulting tsunami would pile a wave more than 20 feet high crashing onto the Oregon-Washington coast, inundating Seattle and the entire Puget Sound region as well as Portland and the mouth of the Columbia River.

Crescent City in California's Del Norte County -- where a smaller tsunami killed 11 people in 1964 after a magnitude 9 Alaska quake -- would see a wave of more than 11 feet, and the tsunami sweeping the coast at the Golden Gate and Monterey Bay would be more than 10 feet . At Santa Barbara, Ward calculates, the wave height would be 6.5 feet, and smaller waves would crash against the shore as far south as the tip of Baja California.

"These calculations are still rough," Ward concedes, "but they do indicate a level of danger that needs to be considered."

The evidence of the great temblor 300 years ago was discovered along the coast of Washington and Oregon by Brian Atwater, a U.S. Geological Survey scientist in Seattle. And Japanese scientists deciphering old tsunami records in their coastal towns calculated that the event had sent a major wave speeding across the Pacific in 10 hours to damage many coastal villages on Honshu, Japan's main island.

Another giant earthquake is nearly a certainty in the unstable coastal regions of Oregon and Washington, but many scientists are also considering the effect of an event that would have no precedent in recorded history -- and have concluded that an even greater tsunami might be generated if an asteroid were ever to plunge into the ocean off the West Coast.