1. How Does the 300-Year Silence Fit into Mount Fuji's History of About 180 Eruptions?
According to materials from the Mount Fuji Hazard Map Study Committee published by Yamanashi Prefecture, about 180 eruptions have been confirmed on Mount Fuji over the past 5600 years [1]. Dividing 5600 by 180 gives approximately 31, so the average rate is about one eruption every 30 years. However, this is an averaged figure; actual eruptions did not occur in a straight line every 30 years.
The last confirmed eruption was the Hoei eruption of 1707, and no eruptions have been confirmed since then for about 300 years [1]. Compared to the average pace, this quiet period is about 10 times longer. This calculation highlights how unusual the current silence is relative to the historical average.
Mount Fuji was born about 100,000 years ago. Since at least 8000 years ago, it has grown through repeated small-to-medium eruptions that piled up lava and volcanic ash on its slopes [2]. The mountain's current quiet appearance is just one scene in that long history.
2. What Happened During the 1707 Hoei Eruption of Mount Fuji?
According to a report by the Cabinet Office, the Hoei eruption began on November 23, 1707 (December 16 in the Western calendar) on the southeast slope. It continued intermittently for 16 days until the early morning of December 9 (January 1, 1708) [2]. Volcanic ash and volcanic lapilli (small volcanic rocks) were carried by westerly winds, falling not only on Edo (modern-day Tokyo) but also on the Boso Peninsula, more than 100 km (about 62 miles) away.
Among the largest eruptions in the last 8000 years are considered to be the Hoei eruption and the Jogan eruption. However, they were very different in character. The Jogan eruption was a gentle type that flowed lava, while the Hoei eruption was explosive. Most of its ejected material shattered and rose high into the sky with smoke [2]. The total volume of ejected material is estimated at about 1.7 km³.
No direct records of human casualties from this eruption have been found so far. It is thought that this was because climbing was banned in winter and there were no settlements near the crater [2]. Nevertheless, earthquakes that seemed like precursors occurred before this eruption.
3. Why Do Records of Earthquakes Before the 1707 Eruption Help Today's Monitoring?
According to reports, a swarm of earthquakes began on the afternoon of the day before the eruption started. Historical records show that as night fell, the area where shaking was felt expanded [2]. Using these records, scientists estimated the position of rising magma and changes in the earth's crust. This became an important clue for current volcanic prediction observations.
The Hoei eruption began 49 days after the great Hoei Tokai-Nankai earthquakes. While it is possible that the large earthquakes triggered the eruption, the same report states that a simple relationship where a nearby major earthquake always causes an immediate eruption does not necessarily hold true [2].
What people 300 years ago wrote down—what shook, when, and where—is connected to how we view observations today. So, how is Mount Fuji being watched now?
4. Which Five Types of Instruments Monitor Mount Fuji Today?
The Japan Meteorological Agency monitors volcanic activity on Mount Fuji with the cooperation of related organizations. They have installed seismometers, tiltmeters, infrasound sensors (which measure air vibrations), GNSS devices (which use satellites to measure ground movement), and surveillance cameras [4].
On March 15, 2011, a magnitude 6.4 earthquake occurred near the southern part of Mount Fuji. A 2012 report from the National Research Institute for Earth Science and Disaster Resilience stated that no significant changes in low-frequency seismic activity related to this earthquake were observed. Additionally, no abnormal fluctuations considered related to volcanic activity were seen in the tiltmeters [5].
No significant changes related to this earthquake were observed. But what size might the next eruption be?
5. Why Might the Next Mount Fuji Eruption Not Be Small?
In materials from the study committee based on data from the National Institute of Advanced Industrial Science and Technology, eruptions over the past 5600 years are categorized as follows: 86 small-scale (49%), 82 medium-scale (47%), and 7 large-scale (4%) [1]. Small and medium eruptions together make up 96% of the total.
However, the same materials state that just because small and medium eruptions are frequent does not mean the next one will be small. There is also a possibility of a rare large-scale eruption [1]. The style of eruption varies too. Some are gentle flows of lava, while others, like the Hoei eruption, are explosive [2]. The next eruption might not be the same type as Hoei.
Even if we do not know "when," current data allows us to consider broadly "what size and what kind" of eruption might happen. This thinking is reflected in the 2021 hazard map revision.
6. What Changed in the 2021 Mount Fuji Hazard Map?
According to a column by the Shizuoka Prefecture Mount Fuji World Heritage Center, the hazard map was revised in March 2021 for the first time in about 17 years (the first edition was from 2004). The original version was based on eruptions from the past 3200 years. However, the 3200-year period left too many eruptions outside the assumptions, so the period considered was expanded to 5600 years [3].
As a result of widening the time frame, eruptions on the southwest slope—which had few eruptions in the past 3000 years—were added to the assumptions. This includes cases where an eruption near the Fuji Miya city area could send lava flows reaching the city in hours [3]. It means such scenarios are now included, not that they will definitely happen in hours.
Near the summit, thick layers of falling volcanic debris may hide unknown craters (buried craters). Therefore, the entire area within a 4 km (about 2.5 miles) radius from the summit has been added to the range where new craters could form [1].
Acknowledging what is not known and expanding assumptions is how preparations have advanced during this quiet 300 years.
If You Want to Investigate Further
You can try a simple activity at home. Get a paper tape (56 cm long) and represent 1 cm as 100 years. This will show the 5600-year history. The rightmost 3 cm represents the approximately 300 years since the Hoei eruption to now.
The average of one eruption every 30 years corresponds to marks spaced about 3 mm apart. If eruptions followed the average pace, there would be 10 marks in that rightmost 3 cm section. However, actual eruptions are not evenly spaced. Think about the difference between "average" and "actual" on your tape.
For official information, you can read the original materials used in this article on the Japan Meteorological Agency's Mount Fuji page, the Cabinet Office report on the Hoei eruption, and the Shizuoka Prefecture Mount Fuji World Heritage Center column.