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Mt. Fuji eruption could leave up to 10 cms of ash in central Tokyo, severely damage economy

This file photo dated June 8, 2018, shows Mount Fuji looming above the clouds with Lake Yamanaka at the bottom. (Mainichi)

TOKYO -- If Mount Fuji erupts as it did during the Edo period (1603-1868), at least 10 centimeters of ash could accumulate in central Tokyo, according to a simulation conducted by the Meteorological Research Institute.

As the fallout is expected to deal a severe blow to economic activities, the government will establish a panel of experts within the Central Disaster Prevention Council by this fall to study countermeasures, incorporating the institute's estimates in the process.

Experts say roads would be covered with just 1 millimeter of ash, while trains would be forced to stop if as much as 5 millimeters of volcanic sediment builds up. The fallout simulation was carried out by Toshiki Shimbori, senior researcher at the institute's volcanology department. Shimbori calculated 1,096 possible fallout patterns using estimates from the December 1707 eruption as well as daily wind and air pressure data measured from 2015 through 2017 by the Japan Meteorological Agency. Of the total, 36 patterns, or 3 percent, had ash as deep as more than 10 centimeters in the Otemachi district in central Tokyo.

Estimates for a maximum fallout reached 30 centimeters to 1 meter in almost all of Kanagawa Prefecture south of Tokyo, and parts of Tokyo, Shizuoka and Yamanashi prefectures. Up to 30 centimeters of ash was estimated to fall in almost all of Chiba Prefecture as well as parts of Saitama and Ibaraki prefectures.

The areas where ash would fall differ by the season, and parts to the east of the mountain would have deeper fallout because of seasonal winds during winter, while ash would spread in all directions during summer. During spring or fall when southeasterly winds blow from Mount Fuji, ash accumulation in central Tokyo would tend to be deeper.

The institute made a similar simulation about 15 years ago, but it was not as detailed as the latest one incorporating daily data changes, and estimated that the ash fall in central Tokyo would be several centimeters. In the 1707 eruption, the fallout is also estimated to have been several centimeters deep in the area.

Shimbori says the importance of the latest simulation was to review the probability of the maximum fallout at various locations.

The government has not examined detailed countermeasures against large-scale eruptions, but will begin to consider ways to ease the expected large impact on the economy, including effective ways to remove fallen ash.

(Japanese original by Tomohiro Ikeda, Science & Environment News Department)

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