Japan's highest peak Mount Fuji has been placed by a team of researchers in a "critical state" to erupt, spurred by the pressure built underneath it by the 2011 magnitude-9 Tohoku-Oki earthquake.
If this happens, at least 1.2 million people will be affected in the prefectural governments of Shizuoka, Yamanashi and Kanagawa.
However, the research team composed of scientists at the Institute of Earth Sciences in Grenoble, France, with their Japanese counterparts cannot categorically say yet when the eruption will take place or if it is imminent.
The scientists used records from 800 seismic sensors that recorded data after the 2011 magnitude-9 Tohoku-Oki earthquake took place to map geological disturbances in fluid in the earth's crust, caused by seismic waves.
They saw that earthquakes of such magnitude can impact volcanoes and influence just how massive a volcanic eruption can be at a single time.
Using seismic noise, the research team tried to measure the perturbations of the mechanical properties of Earth's crust. Seismic noise is associated with seismic waves caused by ocean swell.
Dr Florent Brenguier, lead author of the publication, told The Guardian that seismic waves travelled a long way around the world several times.
"Their movement makes the earth's crust vibrate and, rather like a shock wave, this produces breaks or cracks in the rock."
After the tsunami occurred in March 2011, a magnitude-6.4 earthquake struck four days after it. And then it was followed by many smaller aftershocks, indicating Mount Fuji was under high pressure.
Scientists saw that the perturbations of the Earth's crust were the greatest under volcanic regions, some 400 kilometres away from the quake's epicentre, especially under Mount Fuji.
"We cannot establish a direct relation of cause and effect between quakes and volcanic eruptions, even if statistically the former lead to an increase in the latter," Brenguier said.
"All we can say is that Mount Fuji is now in a state of pressure, which means it displays a high potential for eruption. The risk is clearly higher."
Such findings, and its lack of clear eruption forecast, has prompted the prefectural governments of Shizuoka, Yamanashi and Kanagawa to adopt a comprehensive evacuation plan should the inevitable eruption of Mount Fuji does occur.
The evacuation plan states
- 750,000 people must leave their homes in 14 municipalities in Shizuoka and Yamanashi owing to lava and pyroclastic flows
- 470,000 people, mainly in Kanagawa, would need to evacuate because of volcanic ash in the air
The plan, Japan Times reported, said 406,000 people are currently in Kanagawa, 62,000 in Shizuoka and 1,000 in Yamanashi.
The health of up to 8.85 million people in the three prefectures will be affected by the expected 2 cm or more volcanic ash fall.
Mount Fuji last erupted in 1707, several weeks after a magnitude 8.7 quake rocked the region.