Mount Kusatsu-Shirane, whose eruption on Jan. 23 killed one person and injured 11 others, is a collective name given to a range of mountains formed as a result of volcanic activities, including Mount Shirane and Mount Motoshirane.
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Since 1800, at least 10 small- to medium-scale phreatic eruptions have been recorded near the summit crater of Mount Shirane, mainly around the Yugama crater lake. Meanwhile, there had been no major volcanic activity on Mount Motoshirane in recent years, allowing an attitude of vigilance toward possible eruptions to diminish among authorities and locals. As the geographical features clearly indicate, however, eruptions larger in scale than the Jan. 23 explosion had occurred in many parts of Mount Kusatsu-Shirane in prehistoric times.
There are at least 15 large and small craters near the summit of Mount Motoshirane, evidence that the volcano had repeatedly erupted in the past. While the Japan Meteorological Agency (JMA) said Mount Motoshirane had last erupted about 3,000 years ago, University of Toyama associate professor Yasuo Ishizaki and other volcanologists say they have confirmed more recent traces of eruptions at the mountain dating back roughly 1,200 to 1,500 years.
"It is possible that small-scale eruptions have since occurred time and again. By looking into eruptions that have been overlooked, we can prepare ourselves for small-scale eruptions like the most recent one," said Ishizaki.
At a news conference on Jan. 23, Makoto Saito, director of the Volcanology Division at the JMA's Seismology and Volcanology Department, said, "It's not that we'd never expected Mount Motoshirane to erupt, but we did assume Mount Shirane was more dangerous." Makoto Miyashita, deputy head of the Volcanology Division, also said, "Our observation system for Mount Motoshirane was weak."
There are three types of volcanic eruptions: magmatic eruptions, phreatic eruptions and phreatomagmatic eruptions. The Jan. 23 explosion is believed to have been a phreatic eruption -- in which underground water heated by magma turns into vapor and, without a place to go, bursts through a crater. While phreatic eruptions tend to be small in scale, they can sometimes send volcanic rocks flying, as seen in the deadly eruption of Mount Ontake in 2014.
"It is necessary to analyze volcanic ash and rocks in order to understand the characteristics of the (Jan. 23) eruption," Ishizaki said.
Magmatic eruptions, in which magma is ejected to the surface of a mountain, are famously seen at the Sakurajima volcano in Kagoshima Prefecture and Mount Unzen in Nagasaki Prefecture. Phreatomagmatic eruptions are caused when underground water comes in contact with magma and expands rapidly before spewing out of craters in an explosive manner.