Tokyo — Former government meteorologist Masatoshi Sagara has become a best-selling author in Japan with a startling prediction that Mt. Fuji will blow up next month.
He says studies of various phenomena indicate clearly the 12,388-foot, long-dormant volcano will explode between Sept. 10 and 15.
Although almost all other experts roundly reject his conclusions, Mr. Sagara's book, ''The Great Eruption of Mount Fuji,'' has sold more than half a million copies in the past six months.
If nothing else, the controversy demonstrates that, despite great strides in technology, it is still difficult to predict potentially disastrous movements of the earth's crust with pinpoint accuracy. Many people in Japan are working on the problem, however.
They have the spur of the well-known theory that earthquakes occur in 60-year cycles, and Sept. 1 is the 60th anniversary of the great Kanto earthquake that killed an estimated 140,000 people in Tokyo and Yokohama.
As recently as May 26 of this year, a major earthquake centered offshore took everyone by surprise in northern Japan, leaving 102 dead, almost all from the resulting tsunami, or tidal wave.
The Meteorological Agency's seismographic section has been steadily planting earthquake sensors in the ground in the notorious earthquake belt between Tokyo and Nagoya. But, according to a spokesman, they are located too far from the nests of quake foci off the coast that have caused most of the trouble.
The agency has a plan by 1985 to provide the capital with a greater degree of advance warning through a line of seismographs dropped onto the sea bottom.
There will be five of these siesmographs carefully laid on an ocean ridge in places up to 4,000 meters deep overlooking the notorious ''Sagami trough'' stretching between the Boso Peninsula on Tokyo's northside to the Sagami Peninsula on the south.
Movement here was the cause of the 1923 earthquake that registered 7.9 on the open-ended Richter scale. Two other disastrous earthquakes occurred at the Boso end of the trough in 1600 and 1703.
The agency already has a similar system of sensors, capable of detecting small vibrations that indicate growing pressure in the earth's crust, installed at another ''black spot'' - the Suruga trough, off Shizuoka prefecture, some 180 kilometers south of Tokyo.
''It's like a pencil. . . . Before it snaps we can feel small intense vibrations. The same applies to the first movements of an earthquake,'' says an agency spokesman.
Scientists are also turning to space-age technology in their search for a foolproof method of earthquake prediction.
Experiments are under way here, for example, of a laser emitting and receiving system that can measure distances between two geographical features, say two mountain peaks, down to the last millimeter. The idea is that earth movements will alter the measurement slightly, providing a good early warning sign.
Another program involves the study of the behavior of catfish, thought to be a good indicator of approaching tremors.
All this, however, could go for nought if the population is not prepared for the forecasted eruption.
There is a growing stress on public anti-disaster education. On Sept. 1, Tokyo and 10 surrounding prefectures will be placed on earthquake alert in a drill expected to involve 16 million people. It will be the fourth in a series of counter-panic exercises organized by the governmental Central Disaster Prevention Council.
The scenario is that an earthquake with a magnitude of 8 on the Richter scale has struck the vulnerable Tokai region, south of Tokyo, at 11 a.m.
A council spokesman explained: ''We want to give as many people as possible a chance to experience a sense of danger so they will be prepared to cope with the real thing.''
Railway and highway traffic will be controlled. The government will mobilize a fleet of about 140 helicopters to monitor congestion, help in evacuation of residents, and provide medical aid to supposedly stricken sites.
Most schools in the nation now conduct regular earthquake drills for their students. A mother commented: ''My young son was so well conditioned that when a small tremor occurred while we were having dinner one night, he immediately dived under the table.''
The government also has a mobile home that tours the country showing the public just what happens in a big shake-up. Participants are invited to sit in the living room while giant vibrators create earthquake conditions of various magnitudes.
A dramatic film often screened on television of this experience shows furniture toppling and windows shattering. The usual message is: keep cool, turn off the gas, and find shelter until it is safe to move.
The Tokyo metropolitan government, meanwhile, is working out a long-range disaster prevention plan aimed at minimizing damage through a long-term program that could cost as much as $6 billion.
It calls for development of belts of fire-breaks in overcrowded parts of the city (fires caused the most casualties in the 1923 earthquake), the expansion and improvement of roads, railways, parks, and river banks where people might find shelter, and the fireproofing of buildings along evacuation routes.