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Mt. Fuji eruption could cripple Tokyo with massive amount of ash: disaster simulation

Mount Fuji is pictured in this photo taken from a Mainichi helicopter on Oct. 23, 2019. (Mainichi/Kaho Kitayama)

TOKYO -- A major eruption of Mount Fuji could cripple the Tokyo metropolitan area in just a few hours with up to 490 million cubic meters of volcanic ash, some 10 times the amount of waste generated by the 2011 Great East Japan Earthquake and tsunami, a simulation announced on March 31 by a working group of the government's Central Disaster Management Council has shown.

Depending on the direction of the wind, the ash could bring railway services to a halt over a wide area of the capital sphere, including Tokyo and Kanagawa Prefecture, and also cause power cuts and cut off water supplies, throwing society into disarray.

The simulation was conducted to determine the effect of ash on railway services and lifelines such as electricity. The government is set to begin considering ways to handle such an event alongside relevant ministries and agencies.

In its simulation, the Central Disaster Management Council envisaged an eruption on the scale of the 1707 Hoei eruption of Mount Fuji, producing a large amount of ash that would continue to fall for 15 days. Researchers estimated the effects that different heights of volcanic ash would have, based on similar eruptions that have occurred in the past in Japan and overseas.

Because the areas that would get affected by falling ash would differ depending on the direction of the wind, the council separated its simulation into three scenarios: (1) with the wind blowing strongly from the west, like during the Hoei eruption; (2) with a strong west-southwest wind directly hitting Tokyo, having a major effect on the capital; and (3) with comparatively large changes in the wind, which could also cause damage on the western side of Mount Fuji. The council also mapped the effects of such damage over time with and without rain.

Officials found that in the first two cases, in which ash would fall mainly on the eastern side of Mount Fuji, ash would start to accumulate in the metropolitan area after a few hours. They predicted that even a small amount of ash would cause trains to stop running across the Kanto region in eastern Japan and the central Japan prefectures of Yamanashi and Shizuoka. If the ash were accompanied by rain, 3 millimeters of ash or more would cause electrical wires to short, resulting in power cuts over a wide area of the capital sphere. With water utilities unable to operate, water supplies could be cut, and subway systems could also be halted, the council predicted.

In areas with an accumulation of 30 centimeters of ash or more, accompanied by rain, wooden homes could collapse under the weight.

Up until now, the government had not considered countermeasures to deal with ash in the event of a major eruption of Mount Fuji.

Based on the latest predictions, the Cabinet Office will set up a panel to consider related issues including how the volcanic ash would be removed, as well as potential waste sites, in tandem with relevant ministries and agencies, including the Ministry of Land, Infrastructure, Transport and Tourism.

Toshitsugu Fujii, professor emeritus of the University of Tokyo, who led the working group, commented, "A mistake in the early response could leave tens of millions of people stranded, and it may not be possible to distribute supplies. It's important to prepare a system to handle the situation in advance."

(Japanese original by Mayumi Nobuta, Science & Environment News Department)

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