Some day, possibly soon, a gigantic earthquake will shatter this coastal city and the surrounding region, and if it comes without warning the devastation will be staggering.

Nearly 200,000 homes, a fifth of those standing in Shizuoka Prefecture, will be wholly or partly destroyed. Another 32,000 will be swept away by a huge tidal wave. Many fires will break out, destroying another 250,000 homes as they race quickly among wooden homes and shops. About 11,000 people will die, and 114,000 will be injured.

This is not idle doomsday speculation but the considered judgment of two dozen planners here and many other scholarly experts. Sometime during the next 20 years, they agree, the great Tokai earthquake, named for this coastal region, will shatter the area.

Hiroshi Miyakawa, head of the prefectural post-earthquake planning bureau, discusses it all calmly. Using a map, he points to a spot in nearby Suruga Bay and explains that far below the surface two giant plates of the earth's crust are grinding slowly against each other in a manner known to precede huge quakes.

"We believe it is certain," he says. "The Tokai earthquake will begin soon. It will be before the year 2000, but we think it will not happen in the next three or four years. So we must finish our plans within that time."

Living with imminent disaster would seem as eerie experience, but, like Miyakawa, the Japanese go about it with the stoicism and meticulous planning that they use to manufacture cars and television sets. Planners tabulate probable deaths. Public opinion surveys tap the people's hidden fears. Newspapers carry routine stories on auto jams amid the chaos following a quake.

As little as possible is left to chance. A few hours of warning could make a big difference, so this region on Japan's east-central coast contains more earthquake warning devices than any place except China.

Eighty stations measure tremors on the earth and one cable, fitted with four seismometers, snakes out into Suruga Bay to record movements undersea. The slope of the land is measured because it is known the bay's coastline is sinking rapidily, foreshadowing a massive upheaval.

Water wells are checked routinely because they are known to subside when a quake is coming. Rocks are bombarded with electrons because their resistance to electrons also fore-tells a quake. Underground water is tested for increased density of the element called radon.

Some tests are less scientific. In Tokyo, nine catfish in a tank are watched regularly by marine experts because they are believed to react by sudden movement and erratic jumping at the first sense of tremors coming.

All of these signals -- except those of the catfish, whose credentials as sensors are still in doubt -- are fed to scientists in Tokyo 24 hours a day. If they add up to an imminent quake, six distinguished scholars will be immediately summoned to meet by the pocket beeper they each carry. If they agree on what the signals say, they are to inform the prime minister who will formally announce that the great Tokai earthquake is on its way.

Japan is one of the world's most earthquake-prone countries, and for centuries the people have lived with the knowledge of imminent calamity. Legend has it that Japan's four main islands rest on the back of a giant catfish; when the catfish stirs, the legend goes, the islands are rocked.

Using more modern explanations, experts keep meticulous count of the hundreds of annual tremors caused by shifting subterranean plates. Some experts believe that major quakes occur in specific locations at more or less regular intervals.

One of the six national earthquake experts, Tatsuo Usami of Tokyo University's Earthquake Research Institute, said that the last Tokai region earthquake occurred in 1854. By his calculations another will strike there within the next 20 or 30 years.

Instruments record a severe "strain" in the earth far below Suruga Bay, he said.

"We are near the ultimate strain," he added, which will mean an earthquake of the maximum intensity of 7 on the Japanese scale.

Another sure sign, Usami noted, is the rapid tilting of the earth's surface on the bay's coastline. The coast is sinking while inland areas are rising, a signal that a massive buckling will take place.

For some reason, he added, major earthquakes occur in areas that have been relatively free of tremors in the preceding years. A map of the Tokai region on which prior quakes are noted shows that this particular point of Suruga Bay has been seismically quiet for about 50 years. Usami said it is not known why a peaceful period precedes quakes.

Despite the awareness of oncoming danger, Japanese are traditionally stoic about earthquakes. A public opinion survey of people in Shizuoka Prefecture shows that 70 percent agree a disastrous quake will come upon them sometime, but there is no panicky selling of homes or mass departures.

Even in the current climate of apprehension, when the newspapers are full of potential-disaster stories, severe tremors do not trigger alarmist signs.

More than a hundred tremors, one of them especially severe, swept the area of Izu Peninsula across Suruga Bay from here recently. People went about their business calmly. The tremors were sharp even in Tokyo, and a few people snapped up earthquake survival kits at department stores. Tall buildings swayed sickeningly, but no panic ensued.

But dealing with the public relations of really shattering quakes remains one of the planners' biggest problems. They wonder if announcing the quake in advance when the evidence is clear is the right thing to do. They are worried it could create more panic, hoarding and sudden flight to safe areas.

Miyakawa recalls the experience on Oshima Island in 1978 that followed a major earthquake. The governor announced on television that another big tremor, not nearly so severe, would probably occur quickly. Intended to alert and calm the citizens, it only prompted them to begin evacuating and stockpiling food -- something they had avoided doing before his announcement.