By Taizan Emura / Yomiuri Shimbun Staff WriterThe Jan. 23 eruption of Mt. Motoshirane, in which one person died and another 11 were injured, underscored the threat posed by volcanic activity. The eruption of the volcano, which is part of Mt. Kusatsushirane in Gunma Prefecture, occurred in an area where significant volcanic activity has not been observed for over 1,000 years. Though there are limited financial and human resources that can be allocated to volcanic observation, safety must be ensured through such measures as the provision of detailed information.
Magma reservoirs filled with superheated molten rock lie about 5 to less than 20 kilometers beneath active volcanoes. Eruptions occur when pressure inside a magma reservoir increases to the point that water vapor and magma suddenly eject above ground. There are also eruptions like the one that occurred on Jan. 23, in which heated steam explodes outward.
Many volcanoes show signs before an eruption, such as changes in ground inclination or position. This is because magma reservoirs expand before an eruption.
The movement of liquids such as magma or hot water are also known to cause volcanic tremors that involve continuous minor shaking. When magma or gas starts rising to the surface, rocks may crack and volcanic earthquakes that closely resemble regular earthquakes may occur. If the eruption is small, however, such clear signs are difficult to detect.
In the case of the Mt. Motoshirane eruption, virtually no signs were detected that could have portended an eruption. There were also no cameras to directly capture the eruption.
The Japan Meteorological Agency confirmed the eruption using information from the Tokyo Institute of Technology, which operates a nearby research station, but only did so more than 10 minutes after the event.
Mt. Motoshirane is part of Mt. Kusatsushirane. Scientists have kept a much closer watch on another section, Mt. Shirane, that lies 2.5 kilometers to the north. Mt. Shirane has erupted several times since the Meiji era, so it is monitored around the clock by a network of seismographs and inclinometers positioned around the summit.
Mt. Motoshirane experienced six relatively large eruptions between 1,500 to 5,000 years ago. However, the volcanic activity that took place around 1,500 years ago was not known until recently. There were no noticeable changes to Mt. Motoshirane before Jan. 23, such as the ejection of steam from the ground near the crater.
The mountain is virtually unmonitored. Installing various observation devices around a volcano can cost hundreds of millions of yen, not including subsequent maintenance costs. Prof. Kenji Nogami of the Tokyo Institute of Technology says, “Placing monitoring equipment in quiet locations has never been realistic.”
There are 111 “active volcanoes” in Japan. Some still emit steam, while some show traces of having erupted within the past 10,000 years. For 38 of these volcanoes, the Japan Meteorological Agency, in cooperation with local authorities, has introduced a five-tier “Volcanic Alert Level” system that prescribes behavior for locals and hikers around a volcano.
Most volcanoes are designated Level 1 (Potential for increased activity), the lowest tier on the scale. Six mountains, as of Jan. 26, are designated Level 2 (Do not approach the crater) or Level 3 (Do not approach the volcano) including Mt. Kusatsushirane, which was raised from Level 1 to Level 3 after the recent eruption.
How closely volcanoes are monitored depends on factors such as the frequency of eruptions.
About 330 scientists in Japan are involved in volcano-related research. Some 80 of them actually monitor volcanoes, attempting to forecast eruptions or glean insight into volcanic activity. Yet most such researchers monitor several volcanoes simultaneously, which complicates close observation of all active volcanoes.
Establishing a system to rapidly circulate information during emergencies could help compensate for the difficulty in predicting eruptions. After the eruption of Mt. Ontake on the border of Nagano and Gifu prefectures in September 2014, Yamanashi Prefecture installed a siren system in huts on Mt. Fuji. The system can be activated to warn climbers when the eruption alert level rises or for other situations.
Updating hazard maps would also be an effective measure. The town of Kusatsu in Gunma Prefecture and other entities have created hazard maps for Mt. Kusatsushirane that chart the direction of volcanic mudflows and other information. However, these maps are based on the assumption that Mt. Shirane may erupt. There are references to the eruption of Mt. Motoshirane about 3,000 years ago, which produced lava flows, but the maps do not indicate what areas could be affected by a future eruption or the possible scope of damage.
Hazard maps should include past craters, as it is common for volcanoes to suddenly erupt from long-dormant craters or from nearby mountainsides. Measures to properly convey information to locals and tourists on potential risks should also be considered.