The 2018 phreatic eruption at Mt. Motoshirane of Kusatsu–Shirane volcano, Japan: eruption and intrusion of hydrothermal fluid observed by a borehole tiltmeter network

Earth, Planets and Space

Takahiro Ohkura¹  Hiroshi Aoyama²  Mare Yamamoto³  Taishi Yamada⁴  Tomoki Tsutsui⁴  Yasuo Ogawa⁵  Wataru Kanda⁵  Akihiko Terada⁵  Shin’ya Onizawa⁶ 

[1] Aso Volcanological Laboratory, Kyoto University, 5280 Kawayo, Minami-Aso, 869-1404, Kumamoto, Japan;Institute of Seismology and Volcanology, Faculty of Science, Hokkaido University, N10W8 Kita-ku, 060‑0810, Sapporo, Hokkaido, Japan;Research Center for Prediction of Earthquakes and Volcanic Eruptions, Graduate School of Science, Tohoku University, 6-6 Aramaki-aza-Aoba, Aoba-ku, 980-8578, Sendai, Japan;Sakurajima Volcano Research Center, Disaster Prevention Research Institute, Kyoto University, 1722-19 Sakurajima-Yokoyama, 891-1419, Kagoshima, Japan;Volcanic Fluid Research Center, School of Science, Tokyo Institute of Technology, 2‑12‑1 Ookayama, 152‑8551, Meguro, Tokyo, Japan;Volcanology Research Department, Meteorological Research Institute, Japan Meteorological Agency, 1-1 Nagamine, 305-0052, Tsukuba, Ibaraki, Japan;

关键词: Phreatic eruption;    Tiltmeter;    Kusatsu–Shirane volcano;    Hydrothermal system;    Tensile crack;    Brittle–ductile transition zone;   

DOI  :  10.1186/s40623-021-01475-4

来源: Springer

【 摘 要 】

We estimate the mass and energy budgets for the 2018 phreatic eruption of Mt. Motoshirane on Kusatsu–Shirane volcano, Japan, based on data obtained from a network of eight tiltmeters and weather radar echoes. The tilt records can be explained by a subvertical crack model. Small craters that were formed by previous eruptions are aligned WNW–ESE, which is consistent with the strike of the crack modeled in this study. The direction of maximum compressive stress in this region is horizontal and oriented WNW–ESE, allowing fluid to intrude from depth through a crack with this orientation. Based on the crack model, hypocenter distribution, and MT resistivity structure, we infer that fluid from a hydrothermal reservoir at a depth of 2 km below Kusatsu–Shirane volcano has repeatedly ascended through a pre-existing subvertical crack. The inflation and deflation volumes during the 2018 eruption are estimated to have been 5.1 × 105 and 3.6 × 105 m3, respectively, meaning that 1.5 × 105 m3 of expanded volume formed underground. The total heat associated with the expanded volume is estimated to have been ≥ 1014 J, similar to or exceeding the annual heat released from Yugama Crater Lake of Mt. Shirane and that from the largest eruption during the past 130 year. Although the ejecta mass of the 2018 phreatic eruption was small, the eruption at Mt. Motoshirane was not negligible in terms of the energy budget of Kusatsu–Shirane volcano. A water mass of 0.1–2.0 × 107 kg was discharged as a volcanic cloud, based on weather radar echoes, which is smaller than the mass associated with the deflation. We suggest that underground water acted as a buffer against the sudden intrusion of hydrothermal fluids, absorbing some of the fluid that ascended through the crack.

【 授权许可】

CC BY