ESASHI, Japan, July 14 (1994). They can travel as fast as jet planes. They can carry off entire houses. They can inundate coastal communities with violent flooding. Some English speakers call them tidal waves, but they have nothing to do with tides; much of the world recognizes them by their Japanese name, tsunami.

Huge tsunamis inundated northern Japan Monday night, minutes after a powerful earthquake struck the Sea of Japan west of the northern island of Hokkaido. A tsunami contributed heavily to damage along the coast and to the virtual demolition of the Aonae district on Okushiri, a small island known for fishing and resorts.

People were swept away by huge waves and drowned. Cars were flushed into the sea. Ships were thrown onto land where they crashed into buildings. And hundreds of houses were destroyed in a torrent of water. One of the most striking television images of the quake was that of what looked to be an entire house floating out to sea, its roof protruding above the water. Many things contributed to the damage in the quake, which measured 7.8 on the Richter Scale. There was the shaking, the landslides that ruined roads and buried a hotel, and fires, probably caused by the explosion of ruptured gas lines. But perhaps the most spectacular phenomenon was the tsunami.

Waves Outran Warning
While Japan, perhaps tbe world's most earthquake-prone country, has learned how to build structures to withstand earthquakes, it apparently has not yet been able to fully cope with tsunamis.
"Even wooden houses in Japan are built strong enough to withstand the shaking of an earthquake," said Nobuo Shuto, a professor of tsunami engineering at Tohoku University in Sendai.

Japan has a warning system for tsunamis, but on Monday night the waves reached Okushiri at about the same time as the warning, five minutes after the earthquake. "Under this kind of situation, maybe there is little you can do," Professor Shuto said. "The only way to save human lives is to evacuate immediately, even without a warning."

Waves Up to 35-Feet High
Professor Shuto estimated that the wave that struck Okushiri ranged from 10 to 16 feet high, but noted that he said he had not completed his calculations. A researcher for the Meterologicval Agency estimated, based on a survey of the site, that the waves were as high as 35 feet.
Tsunamis are gigantic versions of the ripples produced by a pebble tossed into a still pond. But in tsunamis, the water is displaced not by a pebble, but by an earthquake, volcanic eruption or other violent undersea movement. A huge mass of water can be displaced.

The tsunami's speed depends on the depth of the water above the displaced sea bed. In the case of the tsunami on Monday, where the water was about 6,000 feet deep, the wave travels at 300 miles per hour.

As the wave approaches the land and the ocean becomes shallower, the water in the back of the wave catches up to the water in the front, and the wave height mounts.

Report Seeing 10 Waves
Depending on the structure of the coastline, a tsunami might strike one time and recede, or reverberate, hitting the shoreline many times. Professor Shuto said some witnesses on Okushiri reported seeing as many as 10 waves.
The professor said that while Japan is most known for tsunamis, they occur elsewhere, and can strike with deadly force thousands of miles from their source. A huge tsunami occurred off the Aleutian Islands of Alaska on April 1,1946, and traveled to Hawaii. Hawaiians thought warnings of a sea disturbance were an April Fool's joke and ignored them, he said, and 159 people were killed.

Japan began constructing defenses against tsunamis after it was hit by what Professor Shuto said was the strongest tidal wave in its recorded history, with waves up to 40 feet high. The tsunami, which struck in 1960 started off the coast of Chile and took 23 hours to cross the Pacific before slamming into Japan's Pacific Coast.