There is a 70 percent probability the Tokyo metropolitan area will be hit directly by a strong earthquake of magnitude-7 level within four years, according to data compiled by a University of Tokyo research team.
The preliminary calculations conducted by a team from the university's Earthquake Research Institute were based on intensified seismic activity in the area after the Great East Japan Earthquake on March 11.
The findings are more dire than a similar estimate by the central government's Headquarters for Earthquake Research Promotion, which states there is a 70 percent chance a quake of the same scale will hit the southern Kanto region, including the metropolitan area, within 30 years.
The central government's Central Disaster Management Council assumes 18 different hypocenters of magnitude-7 level earthquakes, such as in southern Ibaraki Prefecture and the Tachikawa fault zone.
If a magnitude-7.3 earthquake occurs directly under northern Tokyo Bay, as many as 11,000 people are expected to die and about 850,000 buildings to be rendered totally unusable or destroyed by fire.
According to the Meteorological Agency's observations, after the March 11 disaster, earthquakes of magnitude 3 to 6 occurred an average of 1.48 times a day in the metropolitan area through December. This was about five times the pre-disaster average.
Prof. Naoshi Hirata of the university's research institute and others based their calculations on the rule of thumb that the frequency of earthquakes is inversely proportional to their strength. For every increase in magnitude of one, the frequency of their occurrence falls by 90 percent.
The metropolitan area was hit by the magnitude-8 level Great Kanto Earthquake in 1923. Five weaker but still serious earthquakes of magnitude-7 level also hit an area extending from southern Ibaraki Prefecture to the Uraga Channel over a period of about 120 years.
The government's earthquake headquarters obtained its quake estimate data based on the intervals of these quakes in the past. The data did not incorporate the increased seismic activity after the March 11 disaster.
Experts believe seismic activity in the metropolitan area has been intensified by changes in the movements of the Earth's crust since March 11.
"Intensified seismic activity will continue from several to 10 more years," Hirata said. "It's highly probable a strong quake with a magnitude of about 7 will occur during that time.
Daily Yomiuri
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