A handful of Japanese civil defense experts, tiny black boxes in hand, converge on a climate-controlled room. Amid situation maps and the whir of computers, they advise the prime minister to sound a full-scale alert.

Local businessman Jiro Hatano calmly gathers up his stocks of water and dehydrated food and makes a speedy getaway, possibly to a camouflaged concrete bunker in a neighborhood garden, as hordes of his fellow residents of Toyko rush to evacuation staging areas throughout the city.

This doomsday scenario, which specialists say could become a reality any day now, is not based on fear of nuclear attack or invasion but on the threat of the gigantic earthquake that is expected to devastate this congested city of 12.5 million.

Just when calamity will strike is now the subject of a sharp debate among Japanese scientists, who are busily marshaling the country's high technology to the still-murky task of quake prediction. But they generally agree that the city has now entered a danger period, making a major quake likely sometime between now and the year 2000.

"History tells us that Tokyo has been hit repeatedly by giant earthquakes," says Masahiro Kishio, assistant director of the earthquake analysis division at the national meteorological agency. "We can say with absolute certainty that the area will be hit again."

Early in the month, 100,000 residents of Tokyo took part in massive firefighting and evacuation drills in commemoration of the great Kanto earthquake of Sept. 1, 1923. It registered a magnitude of 7.9 on the open-ended Richter scale and killed 143,000 people, mostly in the ensuing firestorm, which reduced the city to a smoldering moonscape.

Should a jolt of similar magnitude rock Tokyo today, government officials estimate that at least 36,000 people would die, and another 63,000 would be seriously injured. Nearly half a million wooden houses and shops would be destroyed by fires or tidal waves, leaving 4 million people homeless.

The officials admit, however, that such calculations are extremely difficult. Tokyo now has five times more inhabitants than at the time of the Kanto quake and a population density of 26,000 people per square mile, or nearly three times that of Washington. Its sprawling cityscape is a maze of skyscrapers, apartment buildings and elevated highways built above hundreds of miles of underground shopping arcades and subway tunnels, all of which has amplified the scope for disaster.

The grim possibilities have sent tremors through Japanese officialdom and touched off massive efforts to shield the old capital from destruction. Tokyo is spending nearly $6 billion -- an amount equal to half its yearly national military budget -- on a current series of countermeasures including the construction of "earthquake-proof" public buildings, hospitals, schools, roads, and sewer and water systems.

More than 1 million rations of milk, rice, hardtack biscuits and a 42-day emergency supply of fresh drinking water have been stocked near five large city parks designated as official evacuation areas. A big budget for television and radio spots and printed materials is aimed at keeping the average resident well briefed on what to do when the big quake comes.

"We can't entirely prevent widespread damage, but we can try to limit it," says Kishio. "And we're fairly certain we can predict the occurrence of a major earthquake within one or two days."

Scientists keep a 24-hour watch over data from devices located at 238 stations along the volcanic spine of the Japanese islands and from one underwater cable on the seabed southwest of Tokyo for signs of the swarm of smaller tremors which are thought to presage a serious jolt.

Tokyo's subterranean water table is checked for a sudden drop that might also indicate a big quake is imminent, while photos from weather satellites are monitored for changes in the cracks in the earth's crust that are thought to run through the city's suburbs.

Should the signs point in an ominous direction, chauffeur-driven government cars will fan out through the city to pick up a half-dozen seismological experts, each fitted out with an electronic signalling device in a black box and deliver them to the situation room at the national meteorological agency. There, they must quickly decide whether to formally ask the prime minister to call a public alert.

A study of a thousand years of historical documents led the late professor Hiroshi Kawasumi of Tokyo University's earthquake research center to the theory that a major quake occurs in Tokyo roughly once every 69 years. Scholars now point out that, according to that theory, the city has already entered a critical period.

According to Hiroaki Yoshii, a senior researcher at the Japan Institute for Future Technology, who has recently completed a study on the subject, a giant-size jolt could create havoc in the country's big business circles because of the high concentration of corporate headquarters in Tokyo. "There would be a sudden shortage of funds from financial institutions to manufacturers, and a lot of business failures and corporate mergers," he says.

Major banks already have begun spreading their highly computerized operations to branch offices around the country and installing emergency communications networks. Earlier plans by the government to relocate the national capital have now been dropped because of the astronomical costs involved, Yoshii says.

Japan, as all schoolchildren here know, is a land of earthquakes and, historically, the destruction brought on by frequent giant tremors has kept a powerful hold on the popular imagination. The Hojoki, a 13th century Japanese classic, says, "For one terror following on another, there is nothing to equal an earthquake."

Today, Japan is jolted by more than a thousand tremors a year large enough to be felt by its inhabitants, many of them in Tokyo. That, and the fact that the city was destroyed both by the great Kanto quake and American firebombings in World War II, has made Tokyo residents largely fatalistic about another impending disaster. And government efforts to gird for the worst, officials complain, have failed to prompt a genuine state of readiness among the public.

"We are the kind of people who spend scads of money and time on vacations and golf," says Hatano, who sponsors a private organization to educate his fellow citizens on earthquake relief measures, "but won't give a second thought to our own individual security."