Ash from Mt. Fuji eruption could block supplies to 60% of Tokyo metropolitan area residents

TOKYO -- When Mount Fuji erupted in 1707, volcanic ash fell for two weeks on the city of Edo, which is part of present-day Tokyo. What would be the worst-case scenario if there is a similar eruption in the future?

According to internal government data on eruption countermeasures obtained by the Mainichi Shimbun, the number of roads blocked by the ashfall increased daily, and it was estimated that by two weeks later, supplies may not reach approximately 60% of the roughly 44.33 million residents of the Tokyo metropolitan area.

For the estimation, the government assumed that multiple major eruptions would occur, similar to the Hoei eruption in 1707, and that volcanic ash would fall in stages for about two weeks in Tokyo, Kanagawa, Yamanashi, Chiba and other prefectures. At a closed-door study session involving relevant ministries and agencies as well as experts, documents showing the results of the calculations were distributed, and specific countermeasures were discussed.

According to the documents, by about two weeks after the eruption, it was estimated that about 27 million residents would be unable to receive supplies because vehicles would be unable to pass due to ash that had accumulated on roads, and some 36 million people would suffer electricity outages caused by the ash that had fallen on power lines.

It was also estimated that up to 26.7 million people would be required to evacuate due to such disruptions and the large amounts of falling ash -- enough to cause wooden houses to collapse.

On the other hand, the key to recovery is how to remove volcanic ash from roads. The internal data estimated that it would take about three days to open one lane of main national roads where emergency vehicles travel, and about five days for two lanes to open, assuming that the number of workers that can be secured is small.

The documents suggested that if roads are not restored, supplies will not be delivered and evacuations may not go as smoothly as expected. In the future, discussion topics will include the possibility of using other means to deliver supplies to areas where they cannot be transported by vehicles.

A Cabinet Office official in charge of volcano disaster prevention said of the internal estimates, "It's just the sum of population data (for areas expected to be affected by ashfall). There has been no progress in examining the data."

Toshitsugu Fujii, professor emeritus at the University of Tokyo, who served as chief of a working group of the government's Central Disaster Management Council that discussed ashfall countermeasures, said, "Three centuries have passed since the Hoei eruption, and it is possible that magma is accumulating. So, there is no surprise that it could erupt at any time."

Fujii then noted, "Depending on how the eruption occurs, traffic in the metropolitan area may be paralyzed, and it is important to discuss how to open roads to provide supplies."

In April 2020, this working group released the results of a hypothetical scenario assuming a major eruption similar to the Hoei eruption, with ashfall lasting for two weeks. Depending on the direction of the wind, ash could fall on central Tokyo within three hours of the eruption, and in some parts of the capital's 23 wards it could reach as much as 3 centimeters in one day and more than 10 cm in two days.

Also, the amount of ashfall that would render two-wheel drive vehicles immobile was estimated to be 3 cm or more during rainfall and 10 cm or more when there is no rainfall. For electric power, it was estimated that more than 3 millimeters of ashfall during rainfall could cause an outage.

(Japanese original by Ikuko Ando, Tokyo City News Department; and Satoshi Yamaguchi and Tomoki Okuyama, Lifestyle, Science & Environment News Department)