A major earthquake with a magnitude of between 7 and 8 is highly likely in most undersea areas along the Japan Trench off eastern Japan within the next 30 years, a government panel said Tuesday.
The government’s Earthquake Research Committee classified as category 3 most undersea areas along the trench, which sits off the Pacific coast from Aomori Prefecture to Ibaraki Prefecture. The classification means the probability of such a major quake is estimated to be 26 percent or higher over the period.
The committee sees an almost zero percent probability of a megaquake similar to the 9.0 earthquake that occurred along the trench on March 11, 2011, and triggered deadly tsunami that hit the Tohoku region. That assessment is unchanged from its previous evaluation, which was released in November 2011.
The committee disclosed the details in an updated version of its long-term quake evaluation of undersea areas along the Japan Trench, which marks a major tectonic subduction zone.
“The possibility of a quake with a magnitude of up to around 8 along the trench remains high,” said Naoshi Hirata, chairman of the committee.
“We want the public to keep in mind that strong quakes and tsunami will hit regions facing the trench again,” he added.
The assessment is based on research conducted since the March 2011 quake.
The earthquake probabilities were calculated based on the number of big quakes seen previously and the intervals between them.
In its most recent evaluation, the committee started classifying undersea areas along the trench into risk categories from 1 to 3. For regions in category 2, the likelihood of an earthquake over the next 30 years ranges from 3 percent to less than 26 percent, while for those in category 1, the probabilities are less than 3 percent.
The committee increased the seismic probabilities listed for two of the 12 possible combinations of magnitude and location along the trench considered prone to earthquakes.
One of the patterns for which the likelihood was raised is a scenario whereby a quake with a magnitude of around 7.9 occurs off Miyagi Prefecture. The committee raised the probability to around 20 percent from almost zero in the previous survey, taking into account the possibility that additional quakes would be triggered by a megaquake.
The probability of a smaller but still significant earthquake occurring off the coast of Fukushima Prefecture with a magnitude of 7 to 7.5 was raised to around 50 percent from some 10 percent.
The possibility of earthquakes with magnitudes measuring from 7 to 7.5 remained high in some undersea areas. Off Aomori Prefecture and northern Iwate Prefecture, the likelihood was estimated at 90 percent or higher. The figure was 90 percent off Miyagi Prefecture and 80 percent off Ibaraki Prefecture.