Over the past half century, Japan has been hit by many volcanic eruptions, but scientifically those were only considered to be medium- or small-scale ones.
While a large-scale eruption has not been experienced in some time, one will likely hit so preparations for such an event have to be made in a hurry.
The Cabinet Office compiled disaster management guidelines to deal with volcanic ash, while the Japan Meteorological Agency (JMA) has decided to introduce a warning system for volcanic ash.
Mount Fuji and other volcanoes have been repeatedly hit by major eruptions and damage can extend to locations far from the volcano.
When Sakurajima in Kagoshima Prefecture on the southern main island of Kyushu erupted in 1914, volcanic ash reached the Kanto and Tohoku regions in eastern Japan.
The Kanto plain suffered major damage when Mount Asamayama in Gunma and Nagano prefectures erupted in 1783.
There are also concerns about multiple damage, such as what occurred in 1707 when Mount Fuji erupted 49 days after an earthquake along the Nankai Trough.
Unlike ordinary ash, volcanic ash consists of sand similar to glass fragments so people’s health will be affected if it gets into their eyes or they breathe it.
Volcanic ash will not naturally disappear and is difficult to remove because it will not wash away even if rain falls. It becomes heavier after absorbing moisture and makes roads slicker.
Social and economic activities will be seriously impacted because volcanic ash will damage electric power facilities and electronic equipment, leading to power outages, stoppages of railway and airline service, communication interruptions as well as damage to the water supply and sewage systems.
Depending on the wind direction during an eruption, the areas to be affected can change vastly and it is also difficult to predict how long the eruption will last.
The guidelines compiled by a panel of experts under the Cabinet Office included a four-stage classification system regarding the need for evacuation should a major eruption occur on Mount Fuji.
While the basic response will be to continue living at home, residents will be asked to evacuate from areas where more than 30 centimeters of volcanic ash falls.
The guidelines are expected to be applied to other active volcanoes around Japan.
The JMA is seeking to begin its warning and advisory system for volcanic ash in a few years.
Advisories will be issued when between 0.1 millimeter and less than 3 cm of volcanic ash accumulates, while a warning will be issued if more than 3 cm, but less than 30 cm accumulates.
A stronger alert will be issued if 30 cm or more accumulates because in such cases there is the danger of wooden homes collapsing in the rain.
If an eruption on the scale of the one in 1707 should hit Mount Fuji, volcanic ash is predicted to fall on 11 prefectures from Shizuoka to Fukushima. The area near Sagamihara in Kanagawa Prefecture is forecast to be hit by more than 30 cm of volcanic ash while the Shinjuku area in central Tokyo could receive more than 3 cm.
Volcanic ash equivalent to 400 Tokyo Domes would have to be removed, a figure that is about 10 times the amount of debris created by the 2011 Great East Japan Earthquake.
Issues that have to be addressed include how to remove that ash and how to dispose of it.
The distribution system will be severely affected so the government is urging residents to stockpile enough supplies to last at least a week.
In addition to preparing for possible earthquakes, strong winds and flooding, there will be a need for measures to deal with volcanic ash, including preparing for protection of the eyes as well as distributing masks and disposing of the accumulated volcanic ash.
Because such a natural disaster has never been experienced after the advanced development of urban areas in Japan, there is a need to hurry in planning and preparing disaster management measures.
The government must also provide information to local governments as well as support their planning efforts.
–The Asahi Shimbun, May 20
AloJapan.com