More than 800,000 buildings would be destroyed and 6.5 million people would have trouble returning home from offices and schools if Tokyo were struck by an earthquake of 6.9 on the Richter scale, the reports said.

The metropolis has a 70 percent chance of being struck by a tremor of 7.0 or higher in the next 30 years, with a quake of 6.9 expected to kill as many as 13,000 people, said the study quoted by the Asahi Shimbun and Kyodo News.

Japan endures about 20 percent of the world's powerful earthquakes. A tremor of 6.1 on the Richter scale rattled the sparsely populated northern island of Hokkaido on Tuesday, the Meteorological Agency said.

The agency revised up its initial reading of 5.8 on the Richter scale and said at least six aftershocks were felt. Police said one woman was slightly hurt.

The government is constantly updating preparations in case a quake hits Tokyo, a city of 12.5 million.

The Asahi said the latest study was the government's most exhaustive yet and would be used for planning how to protect lifelines such as utilities, oil infrastructure, train stations and telephone service.

The research by the Cabinet Office Central Disaster Management Council found that the worst casualties would occur in areas with wooden buildings not resistant to earthquakes, Kyodo News said.

The panel plans to submit the report Wednesday with a final version ready early next year that will also address the economic impact of a major quake, Kyodo said.

The last major tremor to hit Tokyo was the Great Kanto Earthquake of 1923 that left 142,807 dead or unaccounted for.

Japan next month marks the 10th anniversary of the earthquake in the western city of Kobe, which killed 6,433 people.

Experts said the lessons of the Kobe quake helped Japan improve its response time in October when the central Niigata region was hit by a quake of 6.8 on the Richter scale. The quake and hundreds of aftershocks killed 40 people.

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