Towering tsunami up to 34.4 meters would batter Japan's Pacific coastline if an earthquake on the scale of last year's catastrophe struck along the Nankai Trough, according to revised disaster estimates.
The Nankai Trough, an oceanic trench, stretches off the coast of Shizuoka Prefecture to Kyushu.
A government task force considering the impact of a huge offshore earthquake said tsunami 20 meters high or higher would swamp 23 municipalities in six prefectures.
The area includes Omaezaki in Shizuoka Prefecture, where the Hamaoka nuclear power plant is situated.
The panel said an earthquake with an intensity of 7, which is rated as very disastrous and the maximum on the Japanese scale, would affect 153 municipalities in 10 prefectures.
The projected area of devastation is 23 times greater than in the previous study done in 2003.
The new data was compiled by experts appointed by the Cabinet Office.
The task force considered the impact of a monster earthquake in the Nankai Trough, an oceanic trench that stretches off the coast of Shizuoka Prefecture in central Honshu to Kyushu.
The team headed by Katsuyuki Abe, professor emeritus of seismology at the University of Tokyo, based its premise on a magnitude-9.1 quake, similar to the Great East Japan Earthquake of March 11, 2011, occurring.
Under the revised scenario, 687 municipalities in 24 prefectures would experience temblors of at least lower 6 intensity.
The municipalities cover 3.3 times the area of the previous study by the government’s Central Disaster Prevention Council. The 2003 study said 350 municipalities in 20 prefectures would be affected.
The total size of municipalities with temblors of at least upper 6 intensity was put at 5.6 times larger.
The assumed heights of tsunami that would strike prefectures from Ibaraki to Kagoshima were also revised upward.
The number of municipalities that would be hit by tidal waves of 10 meters or higher was increased from a total of 10 in two prefectures to 90 in 11 prefectures, including remote islands under the jurisdiction of Tokyo, where tsunami of 29.7 meters would strike.
The highest tsunami, at 34.4 meters, would swamp the town of Kuroshio in Kochi Prefecture. The height was more than double the previous estimate.
Shizuoka city, with a population of 700,000, could expect tsunami of 10.9 meters in places. That is more than double the previous estimate.
The city of Toyohashi in Aichi Prefecture, which has a population of 380,000, could expect 20.5-meter tsunami.
The projected heights for the prefectural capital cities of Kochi and Miyazaki are 14.7 meters and 14.8 meters, respectively. They are 1.5 to 3 times larger than those of previous estimates.
The task force said that in the three prefectures of Shizuoka, Wakayama and Kochi, it would take only two minutes for the height of tsunami to reach 1 meter after the earthquake.
The corresponding figures for Mie and Tokushima prefectures are three and five minutes, respectively. Those for Aichi and Ehime prefectures are from 10 to 20 minutes.
The task force also projected the maximum heights of tsunami that could strike areas around nuclear power plants.
It said tsunami up to 21 meters could pound the vicinity of the Hamaoka nuclear power plant. That is 3 meters higher than the 18-meter sea wall now being constructed by the plant operator Chubu Electric Power Co. (Chuden) in response to the Fukushima No. 1 nuclear power plant disaster last year.
Chuden may be required to reconsider the height of the sea wall as well as other measures to protect the facility from huge tidal waves.
The 2003 study assumed tsunami with the maximum height of 7.1 meters could hit Omaezaki. The revised figure is nearly 14 meters higher.
The task force said a magnitude-9.1 earthquake could cause ground levels to rise by 2.1 meters, which would mean the 18-meter-high sea wall was still too low to protect the nuclear plant from 21-meter tsunami.
Chuden suspended all reactor operations at the Hamaoka nuclear plant by the end of May 2011 at the request of Naoto Kan, who was then prime minister.
The company expects to complete construction of the sea walls by the end of December. It will then ask the government for permission to restart the reactors.
Shizuoka Governor Heita Kawakatsu was clearly taken aback by the finding, telling a March 31 news conference, "It will seriously influence any decision (on whether to restart operations at the Hamaoka plant)."
Chuden, however, maintains the 18-meter-high sea wall is sufficient to secure the safety of the nuclear power plant even if 21-meter tsunami hit.
"We have already taken a variety of measures to prevent accidents even if the buildings in the compound are inundated with water," an official said.
The government will use the new findings to compile a report on anticipated damage from the earthquake and tsunami by around June.
Local governments are also expected to reassess the strength of embankments, construct more evacuation routes and relocate nursing-care facilities to higher ground.
After the Great East Japan Earthquake, authorities in Kuroshio disclosed plans to relocate the town office from its current location 6 meters above sea level to elevated ground.
Given the new assumption that 34.4-meter tsunami could strike, the town office will review its plan to move to a venue 22 meters above sea level.
Meanwhile, residents of the Uchiuraomosu district in Numazu, Shizuoka Prefecture, are considering a group relocation to a higher ground as many of the 110 households are situated only 4 to 5 meters above sea level.
Under the revised scenario, Numazu could be hit by tsunami with a maximum height of 13.2 meters.
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