The frequency of earthquakes measuring magnitude 3 or stronger in the Tokyo metropolitan area at the end of last year remained higher than before the massive Great East Japan Earthquake on March 11, 2011, academic research showed Sunday.
The study conducted by a group of researchers led by Tohoku University professor Shinji Toda showed that after a rise in the wake of the 2011 9-magnitude quake, the return to a normal frequency has been slower than expected in the Tokyo metropolitan area and also in Kanagawa and Chiba prefectures, an area that could be hit by a major quake.
That suggests the increasing possibility of a huge earthquake hitting the metropolitan area, and also of such frequency remaining high in the next few years, the study said.
"There is a possibility that a phenomenon that cannot be expected after a regular earthquake, may be happening," Toda warned.
According to the research, on average one earthquake a week occurred in the area surveyed before the 2011 mega-quake. But after the quake, the frequency in December 2012 rose to one every two to three days.
Toda said the probability of future earthquake is calculated by a formula based on past quake frequency and, according to the formula, the frequent occurrence of earthquakes of magnitude of 3 or stronger pushes up the probability of a 7-magnitude-level quake in the future.
According to the calculation, there is about a 17 percent possibility that a magnitude of 7 temblor will hit an area centering on Tokyo and Chiba in the next five years, about 2.5 times higher than estimated before 3/11, Toda said.