Japan's tsunami response measures will have to be overhauled to cope with the largest tsunami that can be expected, according to a government panel.
A technical investigation panel under the Central Disaster Management Council said on June 26 that previous planning, based on more conservative estimates, was inadequate.
In the cases of the long-expected Tokai, Tonankai and Nankai earthquakes, the maximum scale envisaged by tsunami response measures is the magnitude 8.6 Hoei Earthquake of 1707, which combined all the three earthquakes.
But seismologists said an offshore focal region close to a trench could move simultaneously, as occurred in the Great East Japan Earthquake, leading to a much larger catastrophe.
In May, Takashi Furumura, a professor at the University of Tokyo's Earthquake Research Institute, announced new estimates.
He said the height of tsunami would be 1.5 to two times the previous estimates if an earthquake similar to the Hoei Earthquake were accompanied by an offshore earthquake similar to the magnitude 7.9 Keicho Earthquake of 1605.
The Central Disaster Management Council's technical investigation panel said countermeasures should focus on how to evacuate residents, instead of relying on seashore protection.
Experts say evacuation routes should be set so that children and elderly residents can evacuate without difficulty and should be easy to follow at night.
In areas without higher ground, buildings or towers should be constructed to accommodate evacuees. The structures should be able to withstand the impact of ships and other heavy debris washed away by tsunami.
Education on disaster prevention and training on evacuation should be reviewed, the council said.
Kansai University professor Yoshiaki Kawata, who chairs the technical investigation panel, said city planning should take into account evacuation routes to allow for passage of large numbers of residents.
Quick and accurate tsunami warnings are vital. In the Great East Japan Earthquake, some people failed to evacuate because the initial warning said the tsunami would be three meters high, compared with the actual average height of 15 meters.
The height of tsunami was underestimated because the scale of the earthquake was assumed to be small.
Experts say methods should be developed to quickly estimate the scale of massive earthquakes, and they say offshore observation capabilities should be strengthened using wave recorders using global positioning systems and undersea water pressure gauges.
The Central Disaster Management Council's technical investigation panel said the central government and local authorities should also prepare for slightly smaller tsunami, which are expected more frequently, or once every 50 to 150 years.
About 60 percent of breakwaters and seawalls in the region hit by the Great East Japan Earthquake, totaling about 190 kilometers of coastline, were damaged. Many structures collapsed after their foundations were weakened by the tsunami.
The technical investigation panel said technologies should be developed to prevent coastal levees from collapsing even when they are hit by tsunami higher than them. If they remain in place, the structures can weaken tsunami and lessen damage. The land ministry is considering ways of reinforcing the foundations of breakwaters and seawalls with concrete.
Shinji Sato, a professor of coastal engineering at the University of Tokyo, said such measures could make it easier to reconstruct areas hit by tsunami by preventing the total devastation seen in some areas on March 11.
A major sticking point for the planners, however, is a delay in the government's reviews of its predictions of the height and damage likely to be inflicted by tsunami.
Local governments are urging the government to conduct the reviews as soon as possible. They cannot start reinforcing coastal levees unless the central government decides on subsidies, allowing them to plan how to finance reinforcement work.
Some local governments have begun to prepare their own forecasts for damage by earthquakes and tsunami.
Shizuoka Prefecture plans to develop a new damage forecast on the assumption that the Tokai, Tonankai and Nankai earthquakes occur simultaneously. But it will not be completed for at least for a year because the prefectural government wants to reflect the central government's predictions in its plan.
The town of Yoshida in Shizuoka Prefecture has begun to prepare a map showing areas threatened by tsunami flooding if the three earthquakes occurred at once.
The town is equipped with a 6-meter coastal levee. Currently, no casualties are anticipated although a 4-meter tsunami is expected to hit the town within 10 minutes if the Tokai Earthquake occurs.
Residents have expressed concerns about the current forecast because of the scale of the Great East Japan Earthquake. The town plans to distribute the map to all households by the end of the year. The map will also show the depths of anticipated flooding and the estimated time of arrival of the first waves.
Mie Prefecture will conduct its own investigations about possible flooding and release the results by September.
It plans to revise its assessment of threatened areas, taking into account the heights of tsunami caused by the Great East Japan Earthquake and past tsunami that have hit the prefecture.
"We can construct large-scale facilities after the central government decides on countermeasures," a prefectural government official said. "But we have to start with what we can do now in the proper way."
Oita Prefecture plans to raise the anticipated heights of tsunami from the Tonankai or Nankai earthquake by 50 to 100 percent.
It will assume a broader focal region than previous studies and also analyze ancient documents and investigate deposit records.
The prefectural government plans to check whether planned locations of evacuation centers are appropriate and put up signs showing the height of various locations in the prefecture.
(This article was compiled from reports by Yusuke Nikaido, Yu Yamada, Shigeko Segawa, Ichiro Matsuo and Yasushi Okubo.)
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