The government’s revised quake-tsunami estimates have sent shock waves across Japan and forced local municipalities to reassess their worst-case scenarios.
The new estimates, based on a tremendous earthquake striking in the 750-km Nankai Trough running from Honshu to Kyushu, warn of devastation far worse than last year’s megaquake and tsunami.
A magnitude 9 quake in the trough would create higher and faster tsunami than those of March 11 and kill tens of thousands — possibly hundreds of thousands — of people in the Chubu and Shikoku regions and destroy several coastal towns.
Since the affected areas would also include major industrial cities, such as Hamamatsu, Shizuoka Prefecture, and Toyohashi, Aichi Prefecture, experts are warning that the economic impact would exceed that of the Great East Japan Earthquake.
One town in the target zone is Kuroshio, Kochi Prefecture, which has been warned that it could be battered by a 34-meter tsunami. About 80 percent of its 12,800 residents live on flatland no more than 20 meters above sea level.
“This is a dangerous situation for the survival of our town,” said Mayor Katsuya Onishi.
Ironically, the town has just completed a 20-meter evacuation tower based on the previous tsunami estimate: 10 meters.
The mayor said the town may now have to consider moving residential areas to higher ground.
The revised report by the Central Disaster Management Council emphasized that the odds of the worst-case scenario coming true are slim, and that it intentionally chose extreme conditions from the dozens of possible tsunami-quake scenarios for each sample town.
But even the more conservative scenarios laid out in a previous government report compiled in 2003 — before last year’s disasters — warned that as many as 28,000 people would be killed by a monster Nankai Trough quake. The March 11 disasters left more than 15,800 dead and 3,000 missing in the Tohoku region as of April 4.
Mayor Naoki Ishida of Shimoda, Shizuoka Prefecture, has decided to push for a plan to relocate City Hall to a site 50 meters above sea level. This was after the latest report said his city of 26,000 people could expect a 25-meter tsunami, about five times higher than one Shizuoka Prefecture estimated in 2001.
Shimoda and its residents had already been discussing whether to move City Hall, a 55-year-old building that is set to be demolished and rebuilt. Before March 11, a quarter of the residents wanted to keep it where it is.
“The worst-case scenario has become an expected possibility. We can’t really ignore it,” said Takahiro Shinji, who is in charge of the relocation plan.
According to the 2012 estimates, a tsunami over 20 meters high could swamp 23 municipalities across six prefectures. The previous report said the tsunami would max out at 17 meters.
Faster tsunami would hike the death toll by giving people less time to flee. Last year’s tsunami reached Tohoku’s coast in 20 to 30 minutes, but the Nankai Trough report estimates that the first wave could hit six prefectures in as little as five minutes, since its epicenter would be much closer.
Making things worse would be the quake itself, which is expected to create public havoc and slow any evacuation attempt.
The new version estimates that an earthquake registering more than lower 6 on the Japanese intensity scale, the same level felt in most of the Tohoku region, could strike 678 municipalities in 24 prefectures, or 7 percent of Japanese soil.
The implications are ominous.
Yoshiaki Kawata, who was in charge of the 2003 government report, warned that the death toll in the latest scenario could reach 300,000, based on his own rough estimates and assuming that no more than 70 percent of the residents would escape, as was the case in Tohoku.
Kawata, a professor of disaster management at Kansai University, is also a key member of a government panel estimating the possible loss of life that would result from the events assumed in the latest worst-case scenario.
Kawata warned that it would be dangerous to focus only on tsunami height. He said the waves can be deadly at a height of just 2 meters, a scale used by the panel to project damage. More than 80 percent of people in the path of a 4-meter tsunami are expected to die if they fail to evacuate.
“It makes no difference if a tsunami is 30 meters or 10 meters high. A 2-meter tsunami can kill you if you do not evacuate,” Kawata said.
The grim forecast is a result of the lessons learned from last year’s disasters.
To brace for the worst and eliminate the unexpected, the government conducted quake-tsunami simulations based on conditions similar to those that emerged on March 11 last year, many of which were never expected to materialize before the Great East Japan Earthquake.
Experts under the Central Disaster Management Council worked under the assumption that several earthquakes would occur simultaneously, even in the least likely areas.
They added the fault around the estuary of the Fuji River in Shizuoka Prefecture, and the area southwest of the Sea of Hyuga off Miyazaki Prefecture. It also took into consideration that tsunami could be spawned by quakes as shallow as 10 km beneath the sea floor.
The panel raised the magnitude of the earthquakes from 8.7 to 9.0, the same as on March 11. It also examined 11 patterns of tsunami and five patterns of earthquakes and picked the worst case for each sample town. These changes in methodology doubled the areas that would affected by a Nankai Trough catastrophe.
These revisions are another example of Japan’s experiments with earthquake prediction. Especially since last year, the relentless quest to figure out when and where the next temblor will strike has been stepped up, only to conclude that it is impossible to pinpoint.
Masakatsu Abe, a professor emeritus at the University of Tokyo and chairman of the expert panel that complied the report, acknowledged that reality can defy the estimates.
“The real earthquake and tsunami could be bigger or smaller,” Abe said at a news conference after the report was released on March 31.
Experts also stressed that the latest estimates are focused only on scenarios triggered by Nankai Trough earthquakes and are not the most probable disasters that could occur in the near future.
“An unexpected scenario could still happen,” said Kawata.
The government panel is slated to compile a comprehensive report detailing casualties and economic damage by autumn.