If big quake hits off coast, tsunami could be gigantic
Geophysicist charts wave heights from Northwest to Baja
David Perlman, Chronicle Science Editor
Monday, June 13, 2005
Debris littered the streets of Crescent City after a tsun... Tsunami Surge. Chronicle graphic by John Blanchard
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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 continentīs western edge -- and now a UC Santa Cruz scientist has calculated the sweep of such an event.
Spurred by the tragedy of Decemberīs 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 Decemberīs 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 Earthīs 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 eventīs 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 Californiaīs 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, Japanīs 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.
Russell "Rusty" Schweickart, the Apollo 9 lunar module commander who is now a retired businessman in Tiburon, has created a foundation with the intention of persuading government agencies to plan for the possibility of an asteroid impact in the ocean -- admittedly, the astronaut says, no more than a 10,000-to-1 chance, but one that could wreak havoc on coastal communities. The specific asteroid that worries him most has been designated by NASA astronomers as 2004MN4, and it is expected to pass within 26,600 miles of Earth less than 25 years from now.
Scientists at NASAīs Near Earth Object Program, which tracks the course of some 70 comets and asteroids that appear to be headed somewhere within thousands of miles of the Earth, calculate that 2004MN4 should make its closest approach to Earth on April 13, 2029, when it will be vividly visible to everyone on Earth. But a collision with Earth is impossible that year, they have reported -- and, they say, "no subsequent Earth encounters in the 21st century are of concern."
Schweickart, however, has concluded there is a remote possibility that the asteroid would collide with Earth in 2036. He and his foundation are urging Congress to send a spacecraft to the asteroid before 2014 to put a radio transponder on the object, which would define the asteroidīs trajectory far more accurately than any other technique.
"While the probability of a highly destructive impact in the immediate future is slight," Schweickart says, "the consequence of such an occurrence is extreme, and mitigation efforts should begin now."
Schweickart enlisted Ward to determine what kind of tsunami might be created if the asteroid did crash in the Pacific in 2036.
And Wardīs calculations indicate a tsunami from the crash would be far more devastating than anything known in history: Peak wave heights, he said, would reach 17 feet in southern Alaska, more than 55 feet all along the California coast, 15 feet in Hawaii, and 20 feet at Puerto Vallarta, the Pacific beach resort in Mexico.
Schweickart maintains that if the transponder were to indicate the objectīs course makes a collision more likely, there could then be time to conceive, plan, design and launch some kind of unspecified "deflection mission. "
"Either way, our course of action is clear," he says. "We either plan another series of cocktail parties to watch the asteroid go by in 2036 -- as we will have done in 2029 -- or we mount the most important space mission in human history."
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