News Mount Fuji eruption: government contingency plans

Last month, internal government data on eruption countermeasures obtained by the Mainichi Shimbun revealed what would happen in case of a Mount Fuji eruption comparable to that of 1707 when volcanic ash fell for two weeks onto the city of Edo. It was estimated that two weeks after such an eruption, 60% of the roughly 44.33 million residents of the Tokyo metropolitan area may be cut off from supplies.

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.

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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.



The biggest issue will be how to remove volcanic ash from roads. 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. If roads are not restored, supplies will not be delivered, and evacuations may not go as smoothly as expected.

The Japanese government now plans to allow the ocean disposal of volcanic ash if a massive ashfall hits the Tokyo metropolitan area.

Mt. Fuji has not erupted for over 300 years since the Hoei eruption and is believed to be in its longest dormant period in the past 5,000 years. When the Hoei eruption started on Dec. 16, 1707, the ash from Mt. Fuji fell for 16 days and reached what is now central Tokyo. The eruption is estimated to have spewed out about 1.7 billion cubic meters of ash and other substances.

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On Friday, a Japanese government panel outlined a set of measures to respond to ashfall from a major eruption of Mt. Fuji, categorising them into four stages according to the severity of the situation.

Stage 1 applies when ashfall is under three centimeters, and there is a risk of railway service suspensions. Ashfall of 3-30 cm will fall under Stage 2 if emergency restoration of lifeline services such as electricity is still possible. Residents will not be asked to evacuate in stages 1 and 2, as danger levels are considered relatively low and due to the complication of organizing a large-scale movement of people in densely populated areas. Stage 3 will be declared when ashfall is 3-30 cm and services like power are cut and unable to be restored quickly. At this stage, local governments may consider evacuating residents to other areas. Ashfall of 30 cm or more will be deemed a Stage 4 incident, and residents will be urged to evacuate. The panel is concerned that wooden buildings could collapse under the weight of ash if combined with rainfall.

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I couldn't help smiling at the levels of severity of ash. The Japanese really love categorising all problems into official levels of severity!

Surely, there are compulsory semiannual ashfall measurement seminars in all municipalities.

I see a pattern: these eruption drill seminars seem to take place annually at the end of March. Municipal budget to burn?

A government and Tokyo metropolitan task force held its first joint meeting with private-sector participants on 25 March to look at how to respond to a possible large-scale eruption of Mount Fuji. The scenario under discussion is one in which volcanic ash spreads across central Tokyo. Such an event could disrupt daily life on a wide scale, and the meeting focused on possible countermeasures.

A simulation video created by the Cabinet Office showed large amounts of volcanic ash falling over the skyscrapers of Shinjuku, illustrating the potential scale of damage if Mount Fuji were to erupt. Estimates indicate that ash accumulation in central Tokyo could reach up to 10 centimeters, raising concerns over widespread disruption, including logistics delays caused by transport paralysis, suspension of railway services, and major outages in electricity and communications. In response, the Cabinet Office and the Tokyo Metropolitan Government convened the meeting with private companies for the first time to coordinate preparedness measures.

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