Hōei Eruption (1707): The Last Eruption of Mount Fuji

The Hōei eruption was the last eruption of Mount Fuji, Japan’s highest mountain. It began on 16 December 1707, only 49 days after one of the largest earthquakes in Japanese history, and for about two weeks blasted ash and pumice across central Japan, burying villages near the volcano and darkening the skies over Edo, the shogun’s capital.

Background: a restless volcano

Mount Fuji, at 3,776 metres, is a stratovolcano built up by repeated eruptions over many thousands of years. Historical records describe significant activity in the eighth and ninth centuries, including the large Jōgan eruption of 864–866, which formed lava fields on the north side of the mountain. After that, eruptions were recorded only occasionally, and by 1700 Fuji had been fairly quiet for centuries.

Fuji was also a sacred mountain. Shinto and Buddhist traditions treated it as the home of deities, and pilgrims climbed it each summer. In the Edo period popular confraternities devoted to Fuji worship were growing among the townspeople of Edo, so the eruption struck a landscape that was both farmland and holy ground.

Japan was then ruled by the Tokugawa shogunate under the fifth shogun, Tokugawa Tsunayoshi. The era name Hōei was adopted in 1704, partly in the hope of a fresh start after the Genroku earthquake of December 1703, which had struck the Kantō region and caused a destructive tsunami and fires. The new era, however, brought further disasters.

The great Hōei earthquake

On 28 October 1707 a massive earthquake struck along the Nankai Trough off the Pacific coast of central and western Japan. Its magnitude is estimated at about 8.6, with modern estimates ranging from roughly 8.4 to 8.7, making it the strongest earthquake recorded in Japan until 2011. A tsunami swept the coasts of Shikoku, the Kii Peninsula and Osaka Bay.

Estimates of the death toll vary widely, from about 5,000 to more than 20,000. Strong aftershocks continued, and in the weeks that followed, people living on Fuji’s lower slopes reported further tremors and rumbling.

The eruption

On the morning of 16 December 1707 (the 23rd day of the 11th month of Hōei 4), after a night of earthquakes, an explosive eruption broke out not from the summit crater but from new vents on the south-eastern flank of the mountain, at around 2,500 to 2,800 metres. A tall eruption column rose above the volcano, and westerly winds carried its ash eastward.

The villages closest to the vents, such as Subashiri, were buried under pumice and ash up to about three metres deep, and houses were set alight by hot falling fragments. The eruption continued in pulses, strongest in its first days, until it ended on about 1 January 1708 (the 8th day of the 12th month). It opened a row of three craters, the largest more than a kilometre across, beside a new hump now called Mount Hōei.

Volcanologists estimate that the eruption produced around 0.7 cubic kilometres of tephra (ash and pumice), measured as solid rock, making it a Plinian eruption of moderate to large size. Unlike many Fuji eruptions, it produced little lava.

The character of the eruption changed as it went on. Its first phase threw out pale, gas-rich pumice of dacite and andesite, which fell as whitish ash. Within hours the material turned to dark basaltic scoria, the type of rock typical of Fuji, and the deposits laid down over the following days are mostly black. Geologists read this change as a sign that a small body of more evolved magma was tapped first, followed by the main basaltic magma beneath the volcano.

Ash over Edo

Edo lay about 100 kilometres to the east. Ash began to fall on the city around midday on the first day. The Confucian scholar and official Arai Hakuseki recorded in his memoir that the sky grew so dark that lamps had to be lit at noon, and that grey ash fell like snow. Over the following days Edo received several centimetres of ash, and many residents suffered from coughs and colds.

Observers in the city also noted the change in colour of the fallout, from light grey at first to black later on, matching the change in the erupted rock. Although the layer in Edo was thin compared with the metres that fell near the volcano, it was blown about by wind for weeks, and contemporary writers complained of dust in houses and streets.

Consequences

The greatest damage was to farmland. In the Sagami and Suruga regions, including much of the Odawara domain, fields were buried under thick layers of ash and could not be planted. Ash washed into rivers raised their beds, and in 1708 the Sakawa River burst its banks, causing floods that recurred for years and spreading the disaster further.

The Odawara domain, ruled by the Ōkubo family, could not support its ruined villages, so the shogunate took the worst-affected lands under its direct administration and compensated the domain with territory elsewhere. In 1708 it also imposed an emergency levy on domains across Japan, at a rate of two gold ryō per 100 koku of assessed land. The levy raised close to 490,000 ryō, but historians estimate that only around a third of it was spent on relief, while the rest helped cover the shogunate’s other expenses.

The official Ina Tadanobu, sent to oversee recovery, is remembered locally for his efforts to help farmers; according to local tradition he released rice from government stores to feed them. Shrines in his honour stand in the area today. Recovery of the farmland took decades, and some villages were not fully restored until the late eighteenth century.

Fuji today

No eruption has occurred since 1708, but Mount Fuji is still classed as an active volcano and is closely monitored. Japanese authorities use the Hōei eruption as the main model for hazard planning: a repeat could bring ash fall to Tokyo that would disrupt transport, power and water supplies for millions of people. Government studies published since the early 2000s, and revised in 2020, use a Hōei-type eruption as their main scenario and warn that even a few centimetres of ash could halt railways and damage power networks in the capital region.

Disasters such as the Laki eruption of 1783 in Iceland show how far the effects of a large eruption can reach.

By the numbers

Timeline

• 1703, December: The Genroku earthquake strikes the Kantō region.
• 1707, 28 October: The great Hōei earthquake and tsunami strike southern and central Japan.
• 1707, mid-December: Earthquakes are felt on Fuji’s slopes.
• 1707, 16 December: The eruption begins on Fuji’s south-eastern flank; ash falls on Edo.
• 1708, 1 January: The eruption ends.
• 1708: The shogunate levies a national relief tax; the Sakawa River floods.

Why it matters

The Hōei eruption is Mount Fuji’s most recent and best-documented eruption. It shows how an earthquake and an eruption can combine into a long-lasting disaster, and it remains the reference point for planning how Japan, and Tokyo above all, would cope if its most famous mountain awoke again.