If volcanic ash falls, what happens to Tokyo?
Mt. Fuji last erupted in 1707. Ash fell on Edo — today’s Tokyo — on and off for 16 days. The government has modeled what a similar eruption would mean for the city today.
Ash is not like snow
Volcanic ash is made of tiny fragments of rock and volcanic glass. It doesn’t melt, its edges are sharp, it becomes heavy when wet — and wet ash conducts electricity. That is why even a thin layer can stop a modern city.
How little it takes
Above-ground railways may stop: ash interferes with signals and track switches.
When ash is wet, outages become possible as it reduces insulation on power equipment.
Two-wheel-drive vehicles can no longer get through: about 3 cm when wet, 10 cm when dry.
Wet ash this deep can collapse wooden houses.
In the government’s worst-case model for a Hōei-scale eruption, central Tokyo could receive up to around 10 cm of ash. Actual amounts depend heavily on the wind on the day.
Ashfall vs. lava flows
Lava and pyroclastic flows
Affect areas close to the mountain in Yamanashi and Shizuoka, and some parts of Kanagawa. People there follow the hazard map and evacuate early.
Ashfall
Travels far with the wind — Tokyo is about 100 km away. Under the national guidelines (2025), people in the ashfall area basically stay at home and live on their stockpiles. Evacuation is advised where wet ash may reach 30 cm, where mudflows are possible after ashfall, and for people who depend on dialysis or care services.
What to prepare
- Masks — ideally particulate respirators (N95 / DS2)
- Goggles. Switch from contact lenses to glasses — ash scratches the eyes
- At least one week of food, water and medicine (the same stockpile as for earthquakes)
- Plastic sheeting and tape to seal vents and cover equipment
- Raincoat, hat, a shovel and sturdy bags for ash cleanup
While ash is falling
- Stay indoors and keep windows and doors shut.
- Avoid driving: visibility drops, roads become slippery, and wipers scratch the windshield.
- Collect ash in bags — don’t wash it into drains, where it sets and causes blockages.
- Clean the roof only when it is safe. Many injuries are falls from roofs.
- Check on neighbors, especially older people and those with respiratory conditions.
Sources
- Cabinet Office — Wide-area ashfall countermeasures for a large-scale eruption, with a Mt. Fuji eruption as a model case (working group report, 2020)
- Cabinet Office — Guidelines for wide-area ashfall countermeasures in the Tokyo metropolitan area (2025)
- Japan Meteorological Agency — Volcanic alert levels for Mt. Fuji
- Mt. Fuji Volcano Disaster Prevention Council — Mt. Fuji Hazard Map (revised March 2021)
General guidance only. In an emergency, follow instructions from JMA, your local government and emergency services.