Vivid portrayal of ash fall damage in Tokyo Metropolitan Area from Mt. Fuji eruption: Cabinet Office releases CG video and calls for preparedness against "national crisis-level" disaster

Mt. Fuji, Japan's highest peak, rises majestically from the earth. Its beautiful conical shape appears different through the seasons, along with the surrounding landscape. While it is a tourism destination known worldwide and visited by many foreign tourists, it is also an active volcano that has erupted repeatedly in the past. Volcano experts point out that "Mt. Fuji has not erupted for more than 300 years, and an eruption could occur at any time."

While awareness has grown that Japan is an "earthquake-prone nation" due to the overwhelming impact of the Great East Japan Earthquake and subsequent major earthquakes, there is less sense of crisis about Japan being an equally "volcano-prone nation." In this context, the Cabinet Office released a CG video on August 26 depicting the damage from ash fall and other effects if a large-scale eruption were to occur at Mt. Fuji.

This video vividly shows how damage from falling ash in the Tokyo metropolitan area would intensify over time, paralyzing urban functions. A Cabinet Office official calls on people to "use this as an opportunity to prepare for a large-scale eruption." We must not forget that a Mt. Fuji eruption is also a "national crisis-level disaster," like a Nankai Trough earthquake or a major earthquake directly hitting Tokyo.

Classified into five types including "large volcanic bombs"

The approximately 10-minute Cabinet Office video includes narration. The video assumes an eruption scale of the same magnitude as the "Hoei Eruption" of 1707, which continued intermittently for 16 days. The specific damage scenarios were created based on a report compiled in April 2020 by the government's Central Disaster Management Council's "Working Group on Wide-area Ashfall Countermeasures in the Event of a Large-Scale Eruption" (Chair: Toshitsugu Fujii, Professor Emeritus, the University of Tokyo).

"Japan is a volcanic nation with 111 volcanoes, and many volcanic disasters have occurred throughout its history. While only relatively small eruptions have occurred in recent years, a large-scale eruption with widespread impacts could occur at any time. Mt. Fuji is no exception." The video released by the Cabinet Office begins with this narration. It then categorizes the specific anticipated damage into five types: "large volcanic bombs" ejected from the crater with the eruption; "lava flows;" "pyroclastic flows;" and "snowmelt-induced volcanic mudflows" flowing down the mountain, and "ash fall affecting wide areas."

Among these, large volcanic bombs are expected to scatter up to 4 kilometers from the crater. In the 2014 Mt. Ontake eruption (in the Nagano Prefecture and the area of the Gifu Prefecture along the prefectural border with the Nagano Prefecture), they scattered about 1 kilometer away, killing 63 people. Lava flows consist of magma that erupts from the crater and flows down the mountain slopes, burning farmland and houses. The projection indicates that these would reach into Kanagawa Prefecture.

Snowmelt-induced volcanic mudflows occur when heat melts snow on mountain slopes, creating large volumes of water that engulf soil and rocks as they flow downward over wide areas. Records show that during the 1926 Mt. Tokachidake (Hokkaido) eruption, two villages were buried and more than 140 people died. Significant damage is also anticipated from a Mt. Fuji eruption.

Volcanic ash consists of fine particles less than 2 millimeters in diameter, composed of mineral crystals and glass particles. In some cases, it can damage eyes or cause health problems through the nose. During the Hoei Eruption, large amounts of ash fell across Edo (present-day Tokyo) for an extended period, reaching as far as the Boso Peninsula.

Wooden house collapse, water quality deterioration, material transport difficulties

The ash fall video also uses the Hoei Eruption as a model case. The impact of ash fall increases over time. Detailed simulations were conducted for distances of approximately 25 kilometers, 60 kilometers, and 100 kilometers from Mt. Fuji's crater.

At the 25-kilometer point, the main substance to fall will be volcanic lapilli (gravel) larger than 2 millimeters in diameter, and in areas closer to the crater, there is a risk of volcanic bombs several centimeters in diameter. Around Sagamihara City in Kanagawa Prefecture, about 60 kilometers from Mt. Fuji, ash resembling beach sand with particles 2 millimeters or less in diameter will begin falling shortly after the eruption, accumulating to about 20 centimeters after two days. In the Shinjuku City of Tokyo Prefecture, about 100 kilometers away, fine ash with particles 0.5 millimeters or less in diameter will fall, reaching a thickness of more than 5 centimeters after two days.

When ash fall of 30 centimeters or more accumulates on the roof of a wooden house and it rains, the combined weight of the volcanic ash and moisture can cause collapse. Sewers and rainwater pipes also become clogged with ash, causing sewage to overflow. In water supply systems, the quality of water deteriorates, facilities lose processing capacity, and there is a risk of water outages.

The video further shows impacts on critical infrastructure. With 3 millimeters or more of ash fall and rain, there is a possibility of power outages due to reduced insulation in insulators. Even trace amounts of ash affect railways, and with 3 centimeters or more plus rain, automobile travel becomes difficult. The video also showed situations where transporting daily necessities becomes difficult.

Additionally, when communications traffic becomes heavy due to safety confirmations and other uses following ash fall damage and the capacity of communication facilities are exceeded, connections become unavailable or difficult. There is also a high possibility of severe damage to crops.

"This quiet state is somewhat abnormal, and it will certainly erupt"

The history of Mt. Fuji's eruptions remains unclear in many respects, but according to the "Mt. Fuji Volcano Disaster Prevention Council," there have been about 180 eruptions over the past 5,600 years. Of these, 96% are said to have been small to medium-scale eruptions. When it comes to confirmed large eruptions with clear records, the "Jogan Eruption" occurred around 864-866, creating the vast Aokigahara forest. The Hoei Eruption occurred after this, and there have been no eruptions since.

Toshitsugu Fujii, who compiled the report that formed the basis of the released video, speaks about Mt. Fuji's eruption risk in the video as follows: "Mt. Fuji was originally a very active volcano, averaging one eruption every 30 years. Recently, it has been in a very quiet state for more than 300 years, which is somewhat abnormal for Mt. Fuji's activity. It has been dormant for more than 10 times longer (than average). The next eruption could occur at any time. Since Mt. Fuji is a young active volcano, it will certainly erupt." The words of one of Japan's leading volcano researchers carry weight.

As Mt. Fuji eruption risks were identified, national and local governments began large-scale activity to prepare countermeasures after 2000. From autumn 2000 to spring 2001, low-frequency earthquakes occurred frequently beneath Mt. Fuji. These earthquakes were analyzed as being caused by the movement of magma-derived fluids, leading to calls for strengthening disaster prevention measures on the premise of an eruption.

In 2012, the Mt. Fuji Volcano Disaster Prevention Council was established, bringing together the government, the three prefectures of Yamanashi, Shizuoka, and Kanagawa, relevant municipalities, and volcano experts, marking the start of serious examination of countermeasures. In 2014, a "Wide-Area Evacuation Plan" was formulated. In 2021, the Mt. Fuji Volcanic Hazard Map was revised, expanding the projected damage zones.

In recent years, in March 2023, the Council revised the evacuation plan for the first time in nine years based on the latest hazard map, compiling a new "Mt. Fuji Volcano Evacuation Basic Plan." Key elements include that residents in areas that will be reached by lava flows within 24 hours should in principle evacuate on foot, and residents who can stay with relatives in distant areas should evacuate voluntarily before an eruption.

One metropolis and ten prefectures: In principle, evacuate if ash fall exceeds 30 centimeters

However, this basic plan for evacuation primarily targets residents of the three prefectures of Yamanashi, Shizuoka, and Kanagawa, who face threats from lava flows and pyroclastic flows, aiming for "zero evacuation delays." It does not anticipate widespread damage in the Tokyo metropolitan area from ash fall. What should residents of the Tokyo metropolitan area outside these three prefectures do?

In response to such concerns, in March of this year, a Cabinet Office expert committee published wide-area ash fall countermeasure guidelines for the Tokyo metropolitan area, stating that if ash fall amounts to "30 centimeters or more," residents should "evacuate in principle." The guidelines cover one metropolis and ten prefectures, including Fukushima and Tochigi Prefectures, which may receive ash fall, including trace amounts.

The guidelines classify responses into four ranks according to the amount of ash fall. Less than 3 centimeters is "Stage 1," 3 centimeters or more but less than 30 centimeters is "2," when damage becomes relatively large (e.g. major power failures) with the same ash fall as "2" it becomes "3," and 30 centimeters or more is "4." "1" and "2" assume situations where infrastructure like railways and electricity/gas are affected but can be restored. People continue living at home and remain within their local area. For "3," when recovery may take a long time, moving outside the home area is also considered.

For "4," with life-threatening ash fall amounts of "30 centimeters or more" where rain would increase the weight of ash and risk wooden house collapse, evacuation is in principle required. Since situations where car evacuation is impossible are conceivable, early-stage evacuation becomes necessary for elderly people, seriously ill patients, and others with difficulty walking. Municipalities are expected to issue calls to residents, and this will also be utilized in creating regional disaster prevention plans.

That said, the timing of evacuation calls is left to municipalities, and there are difficulties as to whether local areas can make accurate judgments for a national crisis-level disaster like a Mt. Fuji eruption. Some form of national/government involvement based on observation information from the Japan Meteorological Agency (JMA) and others seems necessary.

Introduction of "volcanic ash warning" and "advisory"

Currently, operational volcanic disaster prevention information includes "Eruption Warnings" issued according to volcanic alert levels, "eruption flash reports" calling for vigilance among climbers and nearby residents, and, for ash fall, "Ash fall forecasts" of three types: "Regular," "Flash," and "Detailed."

However, ash fall forecasts at most anticipate "1 millimeter or more" and do not address large amounts of ash fall. For this reason, in April, a JMA expert review committee discussing how to disseminate information when large-scale eruptions occur at volcanoes, including Mt. Fuji, compiled a report for the introduction of a "Volcanic ash warning."

According to this report, a "Volcanic ash warning" (tentative name) will be issued when cumulative ash fall of 3 centimeters or more is expected, and a "Volcanic ash advisory" (tentative name) will be issued when 0.1 millimeters or more is expected; both are to be issued by municipality. These are expected to be announced based on ash fall amount even when large-scale eruptions do not occur; for example, advisories are likely to be issued around Sakurajima (Kagoshima Prefecture), which has active volcanic activity.

The report states that when ash fall amounts to "30 centimeters or more," even stronger warnings than the volcanic ash warning are needed, and the JMA plans to examine this. Warnings and advisories are expected to be issued by municipality, and the JMA is preparing for operational launch within a few years, targeting active volcanoes nationwide, not just Mt. Fuji.

According to the JMA 's explanation, current ash fall forecasts predict volcanic ash emission amounts from plume height, also factoring in meteorological data such as wind speed and direction. They announce ash fall areas and the amount predicted using supercomputers. In cases of giant, large-scale eruptions like a Mt. Fuji eruption, the impact is dramatically greater, so not only technology to detect signs of eruption but also technology to improve damage predictions associated with eruptions becomes more important. Budget measures for this are also necessary.

The resolve to replace the ancient "sentimental view of impermanence"

In September last year, the government's Volcano Research Committee at the Headquarters for Volcano Research Promotion published an assessment of the current state of Japan's 111 volcanoes. Of these, it decided to intensively monitor and evaluate eight volcanoes that showed changes in activity status: Mt. Iwate (Iwate Prefecture), Mt. Yake (Nagano and Gifu Prefectures), Sakurajima (Kagoshima Prefecture), Hachimantai (Iwate and Akita Prefectures), Ioto (Tokyo), Satsuma Io Jima (Kagoshima Prefecture), Kuchinoerabu Island (same), and Suwanose Island (same).

Regarding Mt. Fuji, it stated, "activity is calm," and the volcano was not included among these. An eruption does not appear to be imminent. However, volcanic activity can begin with volcanic tremors and low-frequency earthquakes and suddenly become active. Given the huge impact of large-scale eruptions, we should be vigilant.

Japan is one of the world's leading volcanic nations, holding 7% of the world's volcanoes. Visiting Professor Yoshiyuki Tatsumi at the Kobe Ocean-Bottom Exploration Center of Kobe University and a leading Japanese volcanologist alongside Fujii who specializes in magma studies, says that Japanese people have had unique disaster ethics since ancient times.

"Japanese people have been blessed by nature's bounty since ancient times, while also being destined to suffer major natural disasters. From such experiences, a unique perspective on disasters has developed, but we have acknowledged our powerlessness against disasters." Tatsumi spoke thus at a symposium hosted by the Japan Science and Technology Agency (JST) about six years ago. Combined with Buddhism's concept of the "impermanence of all things," this became a "sentimental view of impermanence," he said.

I cannot forget the words he emphasized thereafter: "We must respond to (major volcanic eruptions and giant earthquakes) with an ethical view to replace the view of impermanence. Now is the time to gain the resolve (to live in an earthquake and volcano-prone nation). That resolve is not resignation, but the resolve to confront natural disasters." While there is no probability forecast for Mt. Fuji eruptions like those for giant earthquakes on plate boundaries, the resolve and preparedness to minimize damage from large-scale eruptions that "will certainly come" someday are also required.