A new study is warning that the Tokyo region has a 70 percent chance of being hit directly by a powerful magnitude-7 earthquake within four years.
The study by University of Tokyo seismologists bases the estimate on an increase in earthquake activity in the Tokyo region since last year's March 11 disaster, when a magnitude-9 quake and subsequent tsunami about 140 miles (230 kilometers) northeast of the capital left nearly 20,000 people dead or missing.
The group at the university's earthquake research institute said the number of moderate quakes in the capital region measuring magnitude 3 or bigger surged to 343 in the six-month period after the March quake, up from 47 in the previous six months.
Based on a theory that the probability of bigger earthquakes rises in proportion to an increase in smaller quakes, the team calculated a 98 percent likelihood of a magnitude 6.7 to 7.2 earthquake striking Tokyo over the next 30 years.
Stated differently, "When we ask when a probability of such a quake reaches 70 percent, then we get a 70 percent chance over the next four years," said Shinichi Sakai, a seismologist on the team.
A separate government study estimates that the chance of a magnitude-7 quake striking Tokyo is 70 percent over the next 30 years.
Sakai said the two studies use different methods to calculate earthquake likelihood. While the university study factored in the recent increase in moderate seismic activity, the government estimate only looked the pattern of magnitude 6.7-7.2 quakes over the past 150 years. Since no quake that big has occurred in the Tokyo area since March, there is no change in the government estimate, Sakai said.
Japan is one of the most earthquake-prone countries in the world. Tokyo's last major quake was the 1923 Great Kanto Earthquake that killed 140,000 people.