What Would Happen if Mount Fuji Erupted?

Japan’s Iconic Volcano

Mount Fuji, the tallest mountain in Japan at 3,776 metres, is not only a beloved national symbol but also an active stratovolcano. Its last eruption occurred in 1707 during the Edo period, known as the Hōei eruption, which blanketed nearby areas with ash – even reaching Tokyo (then Edo).

Since then, Mount Fuji has been quiet, but its dormant status doesn’t mean it’s extinct. On the contrary, scientists agree it’s only a matter of time before it erupts again.

The Scientific Warning Signs

Japan’s Meteorological Agency (JMA) closely monitors Mount Fuji with seismic sensors, GPS tracking, and gas emissions data. A 2021 government panel report warned that an eruption today could cause volcanic ash fall across the Tokyo metropolitan area, severely disrupting transportation and infrastructure. A sudden increase in earthquakes or gas emissions near the summit would be early warning signs.

What Could Happen in an Eruption?

Volcanic Ash Fallout

The primary threat from a Mount Fuji eruption is volcanic ash, not lava. Ash particles can:

Collapse roofs due to weight accumulation
Contaminate water supplies
Cause respiratory issues in humans and animals
Disrupt air travel and ground transportation
Severely damage electronic infrastructure

The Tokyo-Yokohama area, home to over 37 million people, lies just 100 km east of Mount Fuji. If prevailing winds push ash in this direction, central Tokyo could receive up to several centimetres of ash within hours.

A 2004 simulation by the Cabinet Office of Japan estimated that in a worst-case scenario:

Up to 15 million people could be affected
Transportation (including Shinkansen lines) would grind to a halt
Power outages could become widespread due to ash interfering with substations and transformers
Economic losses could surpass ¥2.5 trillion (£13 billion) in the first two weeks

Pyroclastic Flows and Lava

Lava flows would likely be slow-moving, but nearby towns such as Fujinomiya, Gotemba, and Fujiyoshida could be at risk depending on the eruption’s intensity. Pyroclastic flows (fast-moving hot gas and rock) would be deadly within a 10–15 km radius, although less likely than ash to reach Tokyo.

Mass Evacuations and Infrastructure Shutdowns

A major eruption could necessitate mass evacuations across Yamanashi, Shizuoka, and parts of Kanagawa Prefecture. Expressways such as the Tōmei Expressway and Chūō Expressway, as well as the Tōkaidō Shinkansen, could be suspended or damaged.

Cities and Towns Most at Risk

City/Town	Est. Distance from Fuji	Potential Risks
Fujinomiya	10–15 km	Lava, ash, pyroclastic flow
Gotemba	20 km	Heavy ash, infrastructure risk
Fujiyoshida	10–15 km	Lava, ash
Kawaguchiko	12 km	Ash fall, tourism impact
Tokyo (Chiyoda)	100 km	Ash disruption, transportation, economy
Yokohama	90 km	Ash fall, transport delays

Japan’s Disaster Preparedness

Japan is globally recognised for its disaster readiness, and Mount Fuji is no exception.

Key Measures Include:

Mount Fuji Hazard Map (Updated 2021): Details lava flow, ash fall predictions and evacuation zones
Volcano Early Warning System: JMA issues alerts in multiple stages (Watch, Warning, Emergency)
Evacuation Plans: Prefectures near Fuji have designated shelters and evacuation routes
Public Education: Schools and municipalities run drills and education campaigns on volcanic risk
Infrastructure Resilience Planning: Railways and power companies have developed protocols for ash-related disruptions

A recent Cabinet Office report urged faster communication systems between government agencies and private sectors to improve coordination during an eruption.

Could It Be Predicted?

While a specific eruption date cannot be forecasted, early warning signs could give several hours to days of notice, depending on the type of eruption. Volcanologists continue to refine models and upgrade equipment to improve predictive capabilities.

The Calm Before the Eruption?

Mount Fuji’s iconic silhouette may appear tranquil, but beneath the surface lies a powerful geological force. While an eruption is not imminent, the consequences of complacency would be devastating. Japan’s thorough preparedness efforts mean that when Mount Fuji does eventually stir, the nation will be as ready as possible.

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