An offshore Pacific earthquake of the scale that hit Japan last year would trigger 34-meter tsunami, resulting in at least 323,000 deaths and devastating much of the coastline from Honshu to Kyushu, experts say.
This grim scenario is the result of a radical reappraisal of a possible magnitude-9.1 earthquake in the Nankai Trough in light of the Great East Japan Earthquake that left 20,000 people dead or missing.
The estimate of fatalities in a Nankai Trough earthquake is 13 times higher than the figure offered by the central government in 2003.
Until the March 11 disaster, experts had assumed that no quake greater than magnitude-8.0 would hit the region off Miyagi Prefecture in northern Japan. Last year's earthquake was magnitude-9.0, putting it among the top five of the world's biggest quakes.
Two expert panels were commissioned by the government to offer a realistic assessment of what Japan can expect if a Nankai Trough earthquake strikes.
At the same time, the panels cautioned that the possibility of such a major disaster occurring was low. However, they called on the public to be "properly fearful" of what may lie ahead.
It is a given that the reappraisal will force the central and local governments to go back to the drawing board in assessing their disaster management plans.
The panels said proper preparations for evacuating residents from towering tsunami caused by a Nankai Trough quake could reduce the estimated fatalities to 61,000, or one-fifth of the level in the worst-case scenario.
The Nankai Trough is an oceanic trench that stretches for about 700 kilometers off the coast of Shizuoka Prefecture in central Honshu to Kyushu. It is a region where quakes frequently occur when the oceanic plate slips under the continental plate.
The grim assessment of possible damage would inevitably be much higher if a Tokai, Tonankai or Nankai quake occurred simultaneously or triggered other quakes along the region.
Much of the damage would come from towering tsunami. The area at risk covers 10.15 million square kilometers, about 1.8 times the area that was inundated after last year's massive earthquake.
One of the panels headed by Katsuyuki Abe, professor emeritus of seismology at the University of Tokyo, estimated the extent of flooding caused by the tsunami produced by the Nankai Trough quake. The other panel, chaired by Yoshiaki Kawata, a professor specializing in disaster management at Kansai University, focused on estimates of fatalities and damage to buildings.
The worst-case scenario was based on a premise of a magnitude-9.1 quake striking. Under the scenario, 151 municipalities in 10 prefectures, ranging from Shizuoka to Miyazaki in Kyushu, would experience shaking of a maximum 7 on the Japanese intensity scale and 239 municipalities in 21 prefectures would experience shaking of upper 6 intensity.
It said tsunami of at least 20 meters would strike eight prefectures, including the islands of the Izu and Ogasawara chains that are under the jurisdiction of the Tokyo metropolitan government.