Ashfall from Mt. Fuji Eruption Included in Evacuation Guidelines; Public Urged to Stay Home If Ash Under 30cm

The Yomiuri Shimbun

17:30 JST, March 21, 2025

Residents in the Tokyo metropolitan area are urged to carry on with life at home when Mt. Fuji erupts but to evacuate if more than 30 centimeters of ashfall accumulates, the government’s expert panel suggested in its draft guidelines released Friday.

The panel to discuss preparations for ashfall in the Tokyo metropolitan area in the event of a Mt. Fuji eruption submitted a report to the Cabinet Office that included a draft of guidelines for refuge actions to be taken by residents.

The Cabinet Office will also apply these guidelines to ashfall from volcanoes other than Mt. Fuji and will notify the relevant ministries and prefectures shortly. This is the first set of guidelines regarding evacuations in the event of ashfall covering extensive areas.

A government forecast in 2020 contemplated a worst-case scenario of ash falling over much of the Tokyo metropolitan area three hours after Mt. Fuji’s eruption, accumulating to a depth of more than 30 centimeters about two weeks later in Kanagawa Prefecture and elsewhere and around 10 centimeters in the center of the capital. The panel came up with specific measures based on this scenario.

The draft guidelines give instructions for basic actions that residents should take according to four stages based on the amount of ash:

From Stages 1 to 3, the panel urges people to stay home and carry on with their activities based on the assumption that ash would not immediately threaten human life. At Stage 3, however, residents are urged to move to a different area in circumstance such as there being a risk of mudslides or a significant impact on daily life, such as major disruptions of transportation or the supply of goods.

At Stage 4, people are urged to evacuate, in principle, against possible mudslides or the collapse of wooden houses by ashfall that becomes heavier when it absorbs rain. In those cases, municipalities should urge people to evacuate outside their residential area.

The guidelines noted the central government will need to provide a forecast for volcanic ash distribution at the time of an eruption so as to help people predict ashfall and take adequate prevention measures. The Japan Meteorological Agency will introduce ashfall forecasts within the next few years following the guidelines and create alerts in the event of a major disaster.

At the time of the Hoei eruption in 1707, ash fell intermittently for 16 days and reached what is now central Tokyo. Since then, Mt. Fuji has been dormant for more than 300 years. However, it has erupted on average once every 30 years over the past 5,600 years, and it could become more active at any time.

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