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Dangerous Waves : Scientists Say the Threat of a Tsunami Is Small but Real

July 25, 1993|KENNETH REICH | TIMES STAFF WRITER

LONG BEACH AREA — Six times in the last 180 years, tsunamis have come ashore on local beaches. But unlike some other places in the Pacific Ocean basin, the waves were comparatively small and the results not catastrophic.

Still, when a major earthquake, followed by seismic sea waves, landslides and fires, caused more than 190 deaths in Japan this month, it raised the question of just how serious the threat of a tsunami coming ashore here is. The National Weather Service, for its part, issued a statement reminding the Los Angeles area of the potential, saying, "Contrary to what some might think, tsunamis do threaten coastal residents of Southern California."

Indeed, in 1930, a tsunami claimed a life in Redondo Beach, and another at Cabrillo Beach in 1960. Also in 1960, hundreds of boats in the Cerritos Channel were cast loose by surging currents when waves arrived from an earthquake in Chile, and $575,000 in damage was done to docks and boats from tsunamis that arrived here six hours after the catastrophic Alaskan earthquake of 1964.

But local experts say the prospects of a really dangerous tsunami striking here remain slim. For instance, Hiroo Kanamori, director of the Caltech Seismological Laboratory, says the recurrence interval of devastating waves here might range from "once every several hundred to even several thousand years."

This contrasts with Japan, where there has been serious loss of life five times in the last century, including more than 20,000 dead from tsunamis in 1896.

A major tsunami here is "very unlikely," says Lucile M. Jones of the Pasadena field office of the U.S. Geological Survey. She adds: "It's not impossible, but it's improbable."

And the state Office of Emergency Services rates the danger so low that when it sought federal funding for a tsunami vulnerability study last year, it confined its request to the Humboldt County coast, where thrust earthquakes in a volcanic subduction zone do pose a threat of major waves at a relatively frequent interval.

"We see the greatest potential there, not on Southland beaches," explained Paul Flores, the office's deputy director.

Records of the National Oceanic and Atmospheric Administration, which has published a book, "United States Tsunamis, 1690-1988," show that since 1812, local beaches have been hit by three locally generated tsunamis and three tsunamis that came from thousands of miles away.

The transoceanic tsunamis came in 1946, 1960 and 1964; the locally generated ones in 1812, 1855 and 1930.

Some of the greatest loss of life worldwide has come from waves generated close to the scene of catastrophe by offshore earthquakes or volcanic eruptions. This was the case in 1883, when 36,000 died from waves caused by the Krakatoa eruption west of the island of Java in the Indonesian archipelago.

But in this area, the worst locally generated tsunami recorded was on Aug. 30, 1930, when a man drowned in Redondo Beach after a magnitude 5.2 earthquake caused undersea landslides in Santa Monica Bay, generating 20-foot waves. Sixteen people had to be rescued that day by lifeguards in Santa Monica.

The most common cause of tsunamis are thrust earthquakes underneath the ocean causing a sudden vertical displacement of a large quantity of water, setting wave motions into effect that race outward from the scene.

Jones points out that, except for a thrust earthquake in the Santa Barbara Channel in 1812, there has not been any example of other thrust faults in the waters immediately off Southern California. And there are no offshore volcanoes here.

That leaves a possibility of landslides from the hills above Malibu, and, as in 1930, underwater landslides from more conventional strike-slip earthquakes, called "slumping."

"There is reason to believe that elsewhere in the world, slumping has been the cause of a few of the major tsunamis," Caltech's Kanamori says. "We really haven't experienced anything on that scale, but at the same time, our history is very short, and there is some danger in relying on our experience because we don't have records before the 19th Century."

As for transoceanic tsunamis, both Jones and Kanamori say that the geometrical alignment of local beaches with the far-off Pacific locales where many tsunamis are generated is not particularly susceptible to the strongest waves hitting here.

"Japan and Chile have a horrible coincidence in terms of their geometry," Jones says. "The sea waves generated by the Chilean earthquake headed straight into Japan and killed 140 people. Here, the alignment was different and the waves not as devastating. . . . Obviously, we can get waves, we're sitting right on the ocean, but the geometry might not be quite right" for the effects to be devastating.

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