TY - JOUR
T1 - The 2018 phreatic eruption at Mt. Motoshirane of Kusatsu–Shirane volcano, Japan
T2 - eruption and intrusion of hydrothermal fluid observed by a borehole tiltmeter network
AU - Terada, Akihiko
AU - Kanda, Wataru
AU - Ogawa, Yasuo
AU - Yamada, Taishi
AU - Yamamoto, Mare
AU - Ohkura, Takahiro
AU - Aoyama, Hiroshi
AU - Tsutsui, Tomoki
AU - Onizawa, Shin’ya
N1 - Funding Information:
This study was supported by the Ministry of Education, Culture, Sports, Science and Technology (MEXT) of Japan, under its Earthquake and Volcano Hazards Observation and Research Program, the Integrated Program for Next Generation Volcano Research and Human Resource Development and Grant-in-Aid for scientific research (No. 17K20141).
Funding Information:
We thank the National Research Institute for Earth Science and Disaster Resilience (NIED) for sharing the tilt records of N.KSHV, N.KSYV, and N.KSNV. The Japan Meteorological Agency provided the tilt records from V.KSAO for the event in 2011. The town of Kusatsu, Agatsuma District Forest Office, Ministry of Agriculture, Forestry and Fisheries, and Ministry of Environment provided the approval for our observations. We are grateful to the Kusatsu Tourism Cooperation for allowing us to use photographs. An early version of the tiltmeter network at Kusatsu–Shirane volcano was established by Jun’ichi Hirabayashi in the 1990s. We thank two anonymous reviewers for their valuable comments. Some figures were drawn with Generic Mapping Tools (Wessel and Smith 1998). We used a 10-m mesh digital elevation model from the Geospatial Information Authority of Japan (GSI) to draw the topographic maps.
Publisher Copyright:
© 2021, The Author(s).
PY - 2021/12
Y1 - 2021/12
N2 - 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. [Figure not available: see fulltext.]
AB - 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. [Figure not available: see fulltext.]
KW - Brittle–ductile transition zone
KW - Hydrothermal system
KW - Kusatsu–Shirane volcano
KW - Phreatic eruption
KW - Tensile crack
KW - Tiltmeter
UR - http://www.scopus.com/inward/record.url?scp=85109689220&partnerID=8YFLogxK
UR - http://www.scopus.com/inward/citedby.url?scp=85109689220&partnerID=8YFLogxK
U2 - 10.1186/s40623-021-01475-4
DO - 10.1186/s40623-021-01475-4
M3 - Article
AN - SCOPUS:85109689220
VL - 73
JO - Earth, Planets and Space
JF - Earth, Planets and Space
SN - 1343-8832
IS - 1
M1 - 157
ER -