TOKYO (AP) _ Watching televised scenes of wreckage today from a devastating earthquake 500 miles to the north, Tokyo residents shivered. In the world's most quake- prone country, everyone fears ''daijishin'' - a great earthquake. The big one.

Monday's quake off the island of Hokkaido, measuring 7.8 on the Richter scale, spawned tidal waves, sparked fires and wiped out whole neighborhoods in coastal communities. Dozens of people were killed and more than 100 missing.

The earthquake, Japan's strongest in nearly a generation, was not even felt in Tokyo. But it sent shock waves of apprehension down the archipelago.

Officials and residents alike know a comparable quake centered near this densely packed capital of 12 million people would dwarf the damage and deaths caused by the northern temblor.

''I try not to think about it, but you can't help it,'' said Yoshi Matsunaga, a Tokyo office worker. ''If it happened here, a lot of people would die.''

The Japanese archipelago is in the world's most active seismic zone, a meeting place of four tectonic plates. The country, with about 10 percent of the globe's seismic activity, experiences more than 1,000 quakes a year.

Japan spends millions of dollars every year on earthquake forecasting and disaster-prevention measures. Critics question whether enough has been done to protect people.

Earthquake-resistant building designs have grown more and more sophisticated, including use of computers and balancing weights to counter the effects of a strong jolt.

But large tracts of land in the Tokyo area have been reclaimed from marshland or the sea, and could liquefy in a major quake. So earthquake engineering alone wouldn't save the buildings on it.

Every neighborhood has a map of open areas where residents can flee, but Tokyo's high land prices prevent the government from buying up large open areas inside the city.

To prevent fires, many homes have computer-controlled switchs to shut off gas with the first strong jolt, and people are constantly reminded to turn off the gas just in case. But many Japanese homes lack central heating and rely on kerosene or gas heaters for warmth that would pose a serious fire danger if a strong quake hit in the wintertime.

This year marks the 70th anniversary of the Great Kanto Earthquake, which destroyed Tokyo in 1923 and killed 140,000 of its then 2 million people.

Some earthquake experts hold that the cycles of major earthquakes in Kanto Plain, where Tokyo is located, run about 70 years - meaning a big one is due now. Others say that calculation doesn't apply to Tokyo itself.

There's a big market in emergency goods. Most department stores sell kits including basic necessities like bottled water and flashlights. More elaborate items can also be found - smokeproof hoods, water purifiers and radios that can be powered with the heat from a candle.

Monday's earthquake struck a relatively lightly populated part of Japan. Hokkaido has about 5.6 million people - less than one-fifth of the 30 million people living in the greater Tokyo metropolitan area.

Estimates of deaths and injuries that would occur in a ''great quake'' in Tokyo - one measuring 8 on the Richter scale - run into the hundreds of thousands. In a survey released this year, the government said the more densely developed central part of Tokyo would suffer the greatest devastation.

The damage would not just be physical. With Japan's formidable economic engine centered in Tokyo, a destructive quake could trigger chaos on world financial markets.

One seismologist, Katsuhiko Ishibashi, has urged decentralization away from Tokyo as the only sure means of keeping down the death toll.

But that's unlikely because the capital is thoroughly entrenched as Japan's administrative, business and cultural center.