Study warns of big Tokyo earthquake
The Tokyo region of Japan has a 70% chance of being hit by a powerful earthquake within four years, a new study has warned.
University of Tokyo seismologists based their estimate on an increase in earthquake activity in the region since last year's March 11 disaster, when a magnitude nine quake and subsequent tsunami about 140 miles (230km) north-east 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 three 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% likelihood of a magnitude 6.7 to 7.2 earthquake striking Tokyo over the next 30 years.
"When we ask when a probability of such a quake reaches 70%, then we get a 70% 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 seven quake striking Tokyo is 70% over the next 30 years.
Mr Sakai said the two studies use different methods to calculate earthquake probability.
While the university study factored in the recent increase in moderate seismic activity, the government estimate only looked at 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, Mr Sakai said.
Japan is one of the most earthquake-prone countries in the world. Tokyo's last major quake was the Great Kanto earthquake in 1923 in which 140,000 people were killed.