News Discussion

Stay Home if Mt Fuji Erupts, New Government Guidelines Advise

On March 21, the Japanese government released its first guidelines for managing volcanic ash fallout in the event of a large-scale eruption of Mt Fuji. The guidelines generally advise residents to remain at home rather than evacuate immediately.

Since ashfall may persist for an extended period, the guidelines suggest storing two weeks' worth of supplies. This is double the amount recommended for a major earthquake directly beneath the Tokyo metropolitan area.

However, the extent and pattern of ashfall will vary depending on factors such as the scale of the eruption and weather conditions. Consequently, local governments and residents may face difficult decisions.

Mt Fuji has erupted approximately 180 times over the past 5,600 years. The last eruption, known as the Hoei Eruption, occurred in 1707.

It lasted intermittently for more than 10 days, sending ash plumes over 10,000 meters high. The eruption ejected an estimated 1.7 billion cubic meters of volcanic material, with ashfall reported as far as present-day northern Chiba Prefecture.

In 2020, the government modeled a scenario similar to the Hoei Eruption, assuming the worst-case conditions with strong southwest winds. Under this scenario, ash accumulation near Sagamihara City, Kanagawa Prefecture, could exceed 20 centimeters by the second day of the eruption. By the 12th day, the ash could approach 30 centimeters.

In Shinjuku Ward, Tokyo, ash accumulation could exceed five centimeters by the second day. By the 12th day, it could approach 10 centimeters.

Ashfall would almost certainly bring public transportation to a standstill. Railway surface lines and airport runways would become unsafe, while roads with as little as three centimeters of ash could become impassable on inclines. The disruption of transportation networks could persist for an extended period, paralyzing the movement of people and goods.

Given these potential disruptions, the guidelines emphasize that residents in affected areas should remain at home rather than evacuate immediately. Historical records show that the Hoei eruption produced continuous ashfall for 16 days. This highlights the need for larger stockpiles than the seven-day supply recommended for earthquakes.

The guidelines also urge local governments and infrastructure operators to develop response plans in advance. These plans should prioritize the restoration and maintenance of lifelines and ensure the rapid supply of essential goods. This is especially important given the expected breakdown of transportation networks.

However, according to government officials, there is currently a disparity between businesses that have made progress in their preparations and those that remain unprepared.

The disposal of volcanic ash presents another major challenge. The total volume of ash requiring disposal is estimated at around 490 million cubic meters. This is more than 10 times the 31 million tons of disaster debris generated by the 2011 Great East Japan Earthquake.

Professor Naoya Sekiya, a disaster sociology expert at the University of Tokyo, warns that modern urban areas are paved, unlike during the Hoei eruption. This prevents the ash from returning to the soil. He cautions that disposal "could take years to complete."

1. 1. evacuate　（危険な場所から安全な場所に）避難する；避難させる
   The hotel guests had to evacuate because of a gas leak.
2. 2. erupt　（火山が）噴火する、爆発する；（感情が）爆発する
   The volcano suddenly erupted, sending ash and smoke into the sky.
3. 3. bring to a standstill　（動き・機能など）を停止させる；（交通機関など）をまひ状態にする
   The heavy snow brought the city to a standstill.
4. 4. Given ～, 　～を考慮すると、～と仮定すると、～を前提として
   Given his experience, he is the best candidate for the job.
5. 5. debris　（破壊された物の）がれき、破片　※不加算名詞
   The rescue team cleared the debris from the collapsed building.

1. 1. How did the Hoei Eruption affect areas far from Mt. Fuji?
2. 2. What problems could ashfall cause for transportation?
3. 3. What actions do the guidelines urge local governments and infrastructure operators to take?
4. 4. What kind of emergency supplies do you keep at home? Do you think two weeks’ worth is enough?
5. 5. If Mt. Fuji erupted today, how do you think it would affect your daily life?

富士山が大規模噴火した際の降灰対策で政府が3月21日に示した初の指針では、住民に自宅などでの生活継続を基本とするよう求めた。降灰はすぐに終わらない可能性があり、首都直下地震などで求める備蓄の倍の「2週間分が望ましい」とも指摘した。「どう降るかは噴火規模や天候による」（有識者）現象だけに、自治体や住民は難しい判断を迫られそうだ。

富士山は過去5600年間に約180回の噴火が確認されているが、1707（宝永4）年の「宝永噴火」以来、起きていない。噴火は10日以上かけて断続的に起き、噴煙は高さ1万メートル以上、噴出物は約17億立方メートルに及んだと推定される。現在の千葉県北部にも降灰したという。

令和2年に政府が示した想定は、宝永噴火と同規模の噴火で被害が最悪となる西南西風が強いケースを設定。相模原市付近の降灰量は噴火2日目に20センチ超、12日目に30センチ近くに及ぶ。東京都新宿区では2日目に5センチ超、12日目に10センチに迫る。

降灰で確実にストップするのが公共交通機関だ。微量でも鉄道の地上路線や空港の滑走路では安全に支障が出る。道路は3センチ以上で上り坂などで通常の走行が難しくなり、各種交通網は長期間にわたり、まひすると考えられる。

このため指針は降灰地域の住民に対し、すぐに避難せず在宅での生活継続を求めた。宝永噴火では降灰が16日間続いたとする記録があり、首都直下地震で求められる7日間よりも多めの備蓄が必要となる。

一方、自治体やインフラ事業者には対応計画の事前策定を求める。交通網の混乱を前提に、ライフラインの復旧・維持や物資供給を急ぐ。「比較的対策を進めている事業者と、未対策の事業者の差がある」（政府関係者）のが実態だという。

除灰の問題も課題となる。処分が必要な火山灰の総量は約4・9億立方メートルと推定され、平成23年の東日本大震災で生じた災害廃棄物約3100万トンの10倍以上となる。検討会メンバーの関谷直也東大教授（災害社会科学）は「宝永噴火の時代と異なり、現代の街は舗装されていて灰は土に返らない。処理に何年もかかる可能性もある」と警鐘を鳴らす。