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Waves of terror.

When an earthquake struck off the coast of Japan this past fall, people in Japan and the U.S. battened down the hatches to await the arrival of a killer tsunami.

On October 4, emergency officials in Neah Bay, Wash., decided to close schools and evacuate the town. "Some people were talking about a [huge] tsunami wave coming from Japan," remembers Charles Arnold, an eighth-grader from the West Coast fishing village. "I thought `Oh, it's just a joke.' But then our principal came in and confirmed that it was real."

With the other students, Charles learned that a magnitude 8.2 earthquake had struck below the seafloor off the coast of Hokkaido, Japan (see map, p. 18). (That's more than 30 times more powerful than either the magnitude 6.7 quake that struck Los Angeles last year, or the 6.9 temblor that hit San Francisco in 1989.) Scientists were worried that the violent undersea upheaval would send a massive seismic sea wave, or tsunami, crashing ashore, not only in Japan, but along the Western U.S., 6,500 kilometers away.

So you think it's impossible for an ocean wave to travel that far? Don't tell that to the residents of Hilo, Hawaii. In 1946, they were hit by a tsunami that began its journey more than 6,000 kilometers away, near Alaska's Aleutian Islands, when a magnitude 7.4 earthquake rocked the seafloor. The massive wave traveled 5 hours before crashing ashore. When it did, it crushed a school, washing students out to sea. In all, 159 people died.


Tsunamis get the energy they need for such long-distance migration and horrific destruction from the undersea earthquakes that trigger them. And large earthquakes are rather common around the "rim" of the Pacific Ocean (see map, below).

That's because the ring of land formations that encircle the Pacific lie at the boundaries of several of Earth's massive slabs of crust called tectonic plates. As these plates grind slowly past, over, or under one another like a bunch of grunting Sumo wrestlers, the moving slabs sometimes "catch." Huge amounts of pressure build up as the plates try to push past one another. Suddenly, when one plate "gives," the release of pressure shakes the earth: an earthquake.

That's what happened near Japan in October. The actual ground shaking killed fewer than 20 people. But many more feared what would follow: When an earthquake strikes under the ocean, it often sends under tsunamis in all directions.

"What happens is the ocean bottom is uplifted," says geophysicist Kenji Satake of the University of Michigan. "The ocean bottom will lift the entire mass of water from bottom to surface, so the entire mass has to move." Try lifting the palms of your hands rapidly out of a basin of water and you'll see what he means.)

A tsunami averaging 4,000 meters deep can cross the Pacific Ocean at jetliner speeds. By contrast, the normal waves you see at the beach, which are caused by wind and tides, travel relatively slowly and are only one to two meter's deep. And where typical ocean waves span a few meters from high point (crest) to high point, a tsunami's wavelength can span hundreds of kilometers.

When these massive swells reach the continental shelf, where the floor of the ocean meets the shore, the upward-sloping bottom may slow the wave to merely hot-rod speeds. But the slope also forces the moving water upward, boosting the wave's amplitude, or height. A tsunami can reach shore as a 100kmph wall of water as high as a house, drowning every meter of beachfront under thousands of tons of water.


That's why officials in Neah Bay, Washington, and Japan weren't taking any chances, says geophysicist Yuichi Nishimura of Japan's Hokkaido University. Japanese schoolchildren learn about tsunami and earthquake safety, says Nishimura. "People living at the seaside know a tsunami is most dangerous." As soon as the October quake struck, Nishimura recalls, people who lived near the shores were evacuated to high ground; ships were also evacuated offshore. "It took less than five minutes after the earthquake occurred to send the highest tsunami alert on TV and radio."

Fortunately, the waves turned out to be less fearsome than predicted. Several three-meter waves crashed ashore in Russian and Japanese fishing villages. But they caused only moderate damage. In Neah Bay, the wave never materialized. Officials there lifted the warning nearly an hour before the 3:30 p.m. predicted arrival time.

Seismologists, scientists who study earthquakes, still aren't sure why the wave never came. One possible explanation: The tectonic plates might have slipped sideways during the quake, releasing some energy without lifting water.

That's just fine with Neah Bay student Charles Arnold. "I was a little bit worried," he says. "We would have had to rebuild our community and town. That's kind of scary."
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Title Annotation:earthquake-driventsunamis
Author:Hart, Stephen
Publication:Science World
Date:Jan 13, 1995
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