Ontake Erupts, Shows Difficulty of Predicting Volcanic Disaster

On September 27, 2014, Mount Ontake suddenly erupted. The 3,067-meter peak, which straddles the border between Nagano and Gifu Prefectures, is Japan’s second-highest volcano behind Mount Fuji and is listed as one of the nation’s “100 Famous Mountains.” Many climbers enjoying the fine weather of the autumn season were caught up in the disaster, which caused numerous fatalities. This is the first time Ontake has erupted since March 2007. The Japan Meteorological Agency has raised the alert level for the volcano to three on its scale of one to five, indicating “Do not approach the volcano.”

One of the Most Volcanic Countries in the World

The JMA defines “active volcanoes” as those that have erupted within the past 10,000 years. There are 110 active volcanoes in Japan, including submarine volcanoes, amounting to 7% of the global total. As one of the most volcanic countries in the world, Japan has abundant hot springs, which represent a major draw at tourist sites nationwide.

According to the JMA, 47 volcanoes, including Mount Ontake, could potentially erupt over the next century. However, the types of eruption and the precursory tremors that serve as warnings vary from volcano to volcano, so there is a limit to how well eruptions can be predicted. The JMA monitors volcanoes around the clock with equipment including seismometers and cameras.

JMA records show that since the eighteenth century, there have been 20 volcanic disasters in Japan resulting in 10 or more dead or missing. The largest of these took place in 1792, when volcanic activities at Mount Unzen (in what is now Nagasaki Prefecture) caused earthquakes and a debris avalanche that killed around 15,000 people.

In the twentieth century, an eruption on the Pacific volcanic island of Torishima in 1902 killed the entire population of 125. In 1914, another powerful eruption killed 58 with subsequent lava flows connecting the former island of Sakurajima to mainland Kyūshū. Then, in 1926, the deadliest volcanic disaster of the century killed 144 people, when the eruption of Mount Tokachi in Hokkaidō melted snow, leading to massive mudflows. More recently, Mount Unzen’s June 1991 eruption caused a pyroclastic flow, a rapid-moving mass of gas and rock fragments, that killed 43 journalists and volcanologists studying the mountain’s activity.

Expert Analysis of the Ontake Eruption

A JMA expert panel on eruption prediction, headed by University of Tokyo Professor Emeritus Fujii Toshitsugu, held a special meeting on September 28 to discuss volcanic activity at Mount Ontake. At the press conference following the meeting, Fujii stated the panel’s opinion that a phreatic eruption, in which heated groundwater becomes steam, had taken place, and that there had also been a pyroclastic flow. “There may be further pyroclastic flows and eruptions,” he warned, with the potential to match the eruption in 1979, when the volcano blasted out 200,000 tons of ash.

The panel explained that the pyroclastic flow had swept more than three kilometers to the southwest, while a plume of ash rose to around seven kilometers above the crater, traveling in an easterly direction. Surveys from the air found that the eruption came from the southwest side of the peak of Kengamine, dispersing large rock fragments within a 1-kilometer radius from the craters. Despite the pyroclastic flow, there were no signs that trees or other vegetation had been burned.

As there was also no evidence of fresh lava among the ash, the panel concluded that the heat of underground magma brought groundwater to boiling point and the rapid increase in pressure caused a phreatic eruption. Minor tremors that began 11 minutes before the eruption continued for around 30 minutes afterward with increased strength. Inclinometers also revealed that the volcano slightly gained elevation from 7 minutes before the eruption and sank again after it took place.

There had also been a large number of tremors earlier in the month, on September 11. But as Fujii said, “In the case of a major eruption, the signs are clear, but it is very difficult to predict eruptions of this smaller scale.”

(Originally written in Japanese on September 29, 2014. Banner photo: A Self-Defense Forces Helicopter flies in front of the erupting Mount Ontake. © Jiji.)