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Giant Waves Often Bring Death and Widespread Devastation : Scientists Baffled By Tsunamis, an Unpredictable Ocean Phenomenon

November 16, 1986|STEWART TAGGART | Associated Press

EWA BEACH, Hawaii — Forty years ago, an earthquake in Alaska's Aleutian Islands unleashed a tidal wave that spread across the Pacific and crashed into Hawaii, killing 164 people and destroying $25 million worth of property.

Tsunami. A Japanese word meaning great harbor wave.

Last May, thousands of people were evacuated to high ground when another earthquake occurred in the same area and warnings were issued for a Pacific-wide tsunami. The waves that arrived in Hawaii were no bigger than those on a good day for surfing.

Hard to Predict

Tsunami. One of the sea's most mysterious and baffling phenomena, difficult to forecast.

"I can tell you more about what we don't know about a tsunami than what we do know," said Gordon Burton, head of the Pacific Tsunami Warning Center, operated by the National Weather Service here as part of an international warning network.

"By the time it hits a coast, it's an unpredictable critter," he added.

When an earthquake measuring 7.7 on the Richter scale was recorded beneath the Aleutian Islands on May 7, the center quickly confirmed a potentially destructive tsunami had been generated, based upon tide station readings at Adak Island. Evacuations were ordered along the coastal areas of Hawaii and the west coasts of the mainland United States and Canada.

Often Imperceptible

Traveling across the ocean at speeds of up to 600 m.p.h., tsunamis are often imperceptible to ships at sea. The waves can have peaks and troughs of just inches, separated by hundreds of miles.

"The Hollywood concept of a tidal wave is a giant wall of water that comes over the horizon," Burton said. "That just could never happen."

It often is not until the wave reaches shallow, shoreline waters that a tsunami can swell into large, destructive waves, usually a series of them. Scientists can estimate when a tsunami will arrive, but not its intensity.

But Burton says officials often err on the side of caution, when the deadly potential is not known.

Only One Warning

In the past five years, about 200 Pacific quakes have been recorded, but during that time the center has issued just two tsunami watches and one tsunami warning. That came on May 7.

The first wave to arrive in Honolulu on that date was followed by three others in quick succession, 11 to 15 minutes apart, a good sign.

Had the waves been farther apart they could have been more destructive, Burton said.

"With those short-term wave trains, you are looking at more rapid oscillations of the sea. There is not the time for it to change drastically in size," Burton said. "A 45-60-minute wave train has much more time to build."

Most tsunamis are linked to strong earthquakes beneath the sea in which one massive tectonic plate suddenly lifts above another. In such areas, known as "subduction zones," the rising plate suddenly displaces a huge mass of water, which in turn creates a tsunami.

'Series of Oscillations'

"Once the ocean is disturbed, it wants to get back to equilibrium, and in doing so goes through a series of oscillations, and that is the wave train that is a tsunami," Burton said.

As the waves radiate out from the source, it may not be the first one that is the most dangerous.

"There can be quiet until the second or third wave comes," Burton said.

Burton said people who put out to sea from Honolulu after the May 7 tsunami warning was issued began returning to shore after the anticipated first wave came in at 5:16 p.m. Many were coming in just as the third and fourth waves arrived, all but unseen.

Severe Shore Currents

The waves can create severe whipsaw-like shoreline currents, throwing boats perilously out of control.

Many residents along the coast of the mainland flocked to shorelines before it was safe, Burton said.

Burton recalls a similar occurrence in Crescent City, Calif., during the Good Friday earthquake of March 28, 1964.

"The downtown pretty much had been inundated by the first couple of waves, and after the second wave had departed, they assumed it was all over," Burton said. "Several people went into downtown, merchants went back to clean up their stores, and several people went back into the local bar to finish their drinks, arriving in time for the third and fourth waves."

Eleven people were killed.

5 Living at Center

Five people live at the Tsunami Warning Center, wearing beepers automatically set off when seismic activity is recorded that could generate a tsunami--earthquakes of 6.5 or larger.

After the May 7 earthquake, Burton said he and others quickly charted the times the tsunami could arrive in Hawaii and on the west coast of North America, and passed the information along to civil defense agencies.

Subduction zones likely to produce destructive Pacific-wide tsunamis are located beneath the Aleutian Islands, the Kamchatka Peninsula in the Soviet Union, and the west coasts of Central and South America.

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