Mt. Fuji Eruption: Preparations Necessary for Widespread Ashfall

Editorial

17:57 JST, June 2, 2025

As a country with many volcanoes, Japan has experienced numerous large-scale eruptions. In modern times, however, there have been no cases in which the Tokyo metropolitan area or other large cities have been blanketed by large amounts of ashfall. It is imperative to make thorough preparations now.

A government panel of experts has compiled a report on measures to be taken against ashfall in the event of a Mt. Fuji eruption. The report is expected to serve as guidelines in the event of large-scale eruptions at not only Mt. Fuji but also other volcanoes across the country.

According to government estimates, volcanic ash could accumulate across a wide area of the Tokyo metropolitan area depending on wind direction. In two weeks, ashfall could reach 30 centimeters or more in Kanagawa Prefecture and around 10 centimeters in central Tokyo.

The report outlined guidelines for action based on the amount of ashfall. If the ashfall is 30 centimeters deep or less, residents should in principle continue their daily lives mainly at home. If the ashfall exceeds that level, the guidelines call for evacuations, again in principle, due to the risk of wooden buildings collapsing under the weight of the ash. Local governments would urge residents to evacuate.

Most people have limited experience with volcanic ash and may find it difficult to envision the nature of the substance. Volcanic ash itself is light, but when it absorbs rain, it becomes heavy, increasing the risk that even 30 centimeters of ashfall could collapse an old house.

Furthermore, volcanic ash conducts electricity more easily when wet. In the event of ash and rain falling around the same time, volcanic ash adheres to electrical wires, causing short circuits and widespread power outages. Railway services are highly likely to come to a halt across a wide area.

Starting in times of non-emergency, industries that support infrastructure such as electricity, communications and railway services should anticipate large-scale ashfall and prepare systems to ensure their operations can continue.

Even if the amount of ash is less than 30 centimeters, caution is necessary for those remaining at home. If 10 centimeters of ash accumulates on roads, vehicles will become unable to move. Even if there is no damage to buildings, disruptions to logistics and transportation could make people’s daily lives difficult. People with chronic illnesses in particular may want to consider evacuating early.

The serious impact on health is also a concern. Volcanic ash contains glass and can damage one’s eyes and throat. It may be useful to add protective goggles and dust masks as disaster prevention items in earthquake kits.

In addition, because volcanic ash does not dissolve in water, it is more difficult to remove than snow. In tandem with measures for dealing with disaster debris, local governments should choose candidate sites in times of non-emergency for the temporary storage of ash.

Based on the guidelines, the Japan Meteorological Agency plans to introduce ashfall forecasts containing relevant advisories and warnings, among other instructions. This is a new system, and it is hoped that the agency will provide information in an easy-to-understand manner.

Mt. Fuji erupted in 1707 during the Hoei era, and what was then Edo was said to be in darkness even during the daytime due to falling ash. Now is the time to recognize the fragility of modern cities, which rely on electricity and automobiles, and the public and private sectors should work together to develop countermeasures.

(From The Yomiuri Shimbun, June 2, 2025)