International Workshop on the mechanism of Phreatic Eruption

Ballistic ejecta of the phreatic eruption at Kusatsu-Shirane volcano, on 23Jan 2018

Mitsuhiro Yoshimoto1 andThe joint research team for ballistic ejecta from the Kusatsu-Shirane 2018 eruption
1 Mount Fuji Research Institute, Yamanashi Prefectural Government
E-mail: myoshi(at)mfri.pref.yamanashi.jp

Mount Moto-Shirane, which is the southern part of Kusatsu-Shirane Volcano in Central Japan,
erupted at about 10.00 AM at 23rd of January 2018 with almost no precursor after 1500 years’
dormancy. The slope of Mount Moto-Shirane is a popular place for ski. During the eruption,some skiers were struck on the slope by the ballistic blocks. One person was died, and other11 people were injured by those ballistic blocks. The ballistic blocks also hit the summit
station of ropeway and the gondola lifts were in operation at the eastern side of the mountain.
When the eruption has started, 50 gondola lifts were still running. A few tens of seconds after,
the ropeway has lost power due to the damage of electrical equipment which were attacked
by ballistic blocks.
We have conducted the survey of the damage situation of the 28 gondola lifts which were inoperation. 15 gondola lifts were covered by ash derived from ash fall and ground-hugging
flow, and 9 of them were seriously damaged, such as window breakage and penetration of
the ceiling by ballistic impacts. Most of the gondola walls that was facing the crater had
cracked and three of the gondola lift also had damages even in the opposite wall. Such kindof damages indicates some of the ballistic blocks fell almost vertically.
We also have carried out a ground-based survey and observational flights using a drone tounderstand the distribution of ballistic rocks. The maximum size of the ballistic rocks wasmeasured at a total of 191 sites and the number density of the rocks (>64mm) was measured
at a total of 64 sites. Ballistic rocks of the eruption are distributed within 500 meters aroundthe main crater and are mainly distributed from north to northeast. The maximum size ofballistic rock was 4.8 m which fell to the western crater rim, and near the main crater was 3.4m. The flight-distance of ballistic rocks with a major axis of about 20 cm, 50 cm, 100 cm, 200cm are 400 to 650 m, 400 m, 350 m, 150 m, respectively. It suggests that the initial velocity
of ballistic rocks of each size is different.

International Workshop on the mechanism of Phreatic Eruption

Fig 1. A. East-southeastern view of new main craters of the eruption taken by A. Terada; B. Photosof ballistic block on the mountain trail 200m away from main crater; C. ballistic block on the ski
course 450m away from main crater; D. damaged situation of Gondola 29Fig.2 Distribution map of maximum block size of ballistic ejecta. Color differentiation indicatesblock size and star is ab14sent of ballistic ejecta.
The joint research team for ballistic ejecta from the Kusatsu-Shirane 2018 eruption：
Mitsuhiro Yoshimoto1, Ryo Honda1, Taisuke Yasuda1, Tomohiro Kubo1, Ryuta Furukawa2,3, Yoshihiro Ishizuka2, Yusuke
Minami2, Masashi NAGAI4, Shuichi Hosokawa3, Akihiko Terada5, Tatsuji Nishizawa5, Yasuhiro Ishimine1,6, Kenichi Arai7,Hisashi Sasaki7, Shino Naruke7, Yuko Sekiguchi3, Takahiro Yanada3, Yasuo Ishizaki8, Nobuko Kametani1,8, Fukashi
Maeno9, Hiroyuki Yamada10(1.Mount Fuji Research Institute, Yamanashi Prefectural Government, 2.Geological Survey of Japan, National Institute ofAdvanced Industrial Science and Technology, 3.Japan Meteorological Agency, 4.Volcano Research Department, National
Research Institute for Earth Science and Disaster Resilience, 5.Volcanic Fluid Research Center, Tokyo Institute ofTechnology, 6.Research and Education Center for Natural Hazards, Kagoshima University, 7.Asia Air Survey Co., Ltd., 8.
Graduate School of Science and Engineering, University of Toyama, 9. Earthquake Research Institute, University of Tokyo,
10. National defense academy)