Earthquakes & volcanos in Japan

Under construction｜2.1.3 How would an eruption of Mt. Fuji affect our lives?

*The contents may be subject to change without prior notice.

K. Z. Nanjo¹⁾ and M. Yoshimoto²⁾

1) NaDiR, Global Center for Asian and Regional Research, University of Shizuoka

2) Mount Fuji Research Institute, Yamanashi Prefectural Government

An eruption of Mt. Fuji could trigger various disasters. Of these disasters, this section focuses on volcanic ash fall, pyroclastic flow, and debris avalanche.

Volcanic ash fall itself does not cause fatal damage, but inhalation of the ash can be detrimental to human health. So, precautions should be taken to avoid inhalation, such as wearing a face mask. Ash fall may also seriously disrupt public services, such as transportation, power supply, water supply, and sewerage system. When Mt. Aso in Kyushu erupted in 2016, volcanic ash, which was made muddy by rain, was deposited on electric power transmission lines and caused short-circuits, resulting in large-scale blackout (power outages). A future eruption of Mt. Fuji is likely to cause similar damage.

As volcanic ash deposited on the ground is powdery, it can be easily carried away by rain and cause a debris flood. In the 1990 eruption of Mt. Unzen-Fugendake (Mt. Unzen or Unzen-dake is an active volcanic group of several overlapping stratovolcanoes and one of the stratovolcanoes is Mt. Fugen or Fugen-dake) in Kyushu, huge quantities of volcanic ash and other ejecta (volcanic products as a result of eruptions) were deposited on the ground, and heavy rain that followed caused a debris flood, washing away houses and cars in the downstream area. Old documents on the Hoei Eruption¹⁾ of Mt. Fuji that spewed volcanic ash into the sky also tell us much about the damage caused by debris flows.

A pyroclastic flow is a fast-moving high-temperature stream of volcanic ash and rock fragments that flows down the slopes of a volcano while mixing with ambient air. The flow moves at such high speed, sometimes exceeding 100 kilometers per hour, that there is no way for us to outrun it. With temperatures between 100°C and 600°C, a little lower than that of lava, inhalation of the hot air from a pyroclastic flow can burn the respiratory tract, making breathing impossible and even resulting in death. It is known from the deposits that remain today that past eruptions of Mt. Fuji induced pyroclastic flows, suggesting the high probability of pyroclastic flows occurring in a future eruption.

Technically, a debris avalanche is triggered by a sector collapse (collapse of a portion of the volcano) induced by an eruption, an earthquake, etc., which cause a portion of the mountain to slide down the slope. If a large-scale debris flood caused by a debris avalanche flows into a river, muddy water is sent downstream, and if it reaches the sea, a tsunami can occur. In the past, debris avalanches have repeatedly occurred on Mt. Fuji. The most recent one was the Gotemba debris avalanche²⁾ (See also Ref. 3) about 3,000 years ago (Fig. 2-7), which caused debris floods to flow from Gotemba to Numazu and Odawara. Due to their rarity, sector collapses are seldom considered in disaster risk management; it suffices to simply know that such an event can occur.

There are few opportunities to learn how volcanic eruptions can affect modern society firsthand, so news of volcanic eruptions in Japan and abroad is an important source of information.

(Fig. 2.7 will aprear here.)

Fig. 2-7: Areas affected by the Gotemba debris avalanche and debris flows that branched off from the avalanche. If large quantities of debris flow into the sea at once, a tsunami may result.

Sources and references