The Hōei great eruption
Mount Fuji's serene, snow-capped cone belies the immense pressures building deep beneath its surface. Its last confirmed eruption, the Hōei great eruption, began on December 16, 1707, and was a violent, multi-day event. It did not produce lava flows but ejected an estimated 800 million cubic meters of volcanic ash. This Plinian-type eruption was powerful enough to blanket Edo (modern-day Tokyo), nearly 100 kilometers away, in a layer of ash several centimeters thick. The eruption was preceded 49 days earlier by the 8.6-magnitude Hōei earthquake, one of the largest in Japan's history. The quake is believed to have compressed the magma chambers deep beneath the volcano, triggering the subsequent blast. The 16-day eruption created a new crater on Fuji's southeastern flank, now known as Hōei-zan, which is a reminder of the event. While there were no recorded deaths directly from the eruption, the widespread ashfall led to crop failures, river damming, and subsequent flooding and starvation.
A critical state of pressure
Historically, Mount Fuji has experienced periods of significant activity, but the interval of over 300 years is unusually long. Scientists are concerned that the volcano is in a dangerous state. Following the magnitude 9.0 Tōhoku earthquake in 2011, studies indicated that the pressure within Fuji's magma chamber has increased significantly. Research by the National Research Institute for Earth Science and Disaster Resilience estimated that the pressure had risen to 1.6 megapascals. This pressure is substantially higher than the threshold believed to have triggered the 1707 Hōei eruption.
A dense network of instruments constantly monitors the volcano for any signs of unrest. The Japan Meteorological Agency and other institutions use seismometers, GPS stations, tiltmeters, and gas monitoring equipment to detect ground deformation and magma movement. In response to the heightened risk, the Japanese government's Central Disaster Management Council, along with the prefectures of Shizuoka, Yamanashi, and Kanagawa, has developed detailed hazard maps and evacuation plans. These plans outline scenarios for evacuating hundreds of thousands of residents in the event of an eruption, with specific protocols based on the volume and type of volcanic threat, from lava flows to ashfall. Should ash accumulation exceed 30 centimeters, for example, evacuation is advised due to the risk of buildings collapsing.