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

Abstract

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 crack azimuth 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 * 10 5 and 3.6 * 10 5 m 3 , respectively, meaning that 1.5 * 10 5 m 3 of expanded volume formed underground. The total heat associated with the expanded volume is estimated to have been ≥10 14 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 yr. Although the ejecta mass of the 2018 phreatic eruption was small, the 2018 MPCG eruption was not negligible in terms of the energy budget of Kusatsu–Shirane volcano. A water mass of 0.1–2.0 * 10 7 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.

Monitoring of magmatic–hydrothermal system by noble gas and carbon isotopic compositions of fumarolic gases

“…The synchronous responses may reflect that both fumaroles are connected to the hydrothermal fluid reservoir that caused the phreatic eruption at Mt. Motoshirane 21 . This is consistent with a magnetotelluric study 23 that suggested the presence of a hydrothermal fluid reservoir (the C2 conductor in Fig.…”

“…2 c). The enhanced volcanic activity may reflect the supply of magmatic fluid from a magma chamber, a potential pressure source for deep inflation at a depth of ~ 4 km since 2018 21 , 26 .…”

“…During the 2018 eruption, fluid injection from the hydrothermal fluid reservoir to shallower depths was suggested from ground deformation recorded by a borehole tiltmeter network at the Kusatsu-Shirane volcano 21 . The low ( 3 He/ 4 He) corr ratios after the eruption might indicate that the hydrothermal fluid in the deeper part of the fluid reservoir is more influenced by crustal He than the shallower part, which constantly supplies fluid to the fumaroles.…”

“…Ainomine, and Mt. Motoshirane, where phreatic eruptions frequently occur 21 . The presence of an electrically conductive structure at depths of 1–3.5 km broadly extending from beneath Mt.…”

Total mass estimate of the January 23, 2018, phreatic eruption of Kusatsu-Shirane Volcano, central Japan

“…Volcanic tremors were recorded for 8 min beginning at 09:59 (Yamada et al 2021). According to the record of the change in the tilt (Terada et al 2021), the area around the MPCG uplifted from 10:00 to 10:02 and subsided from 10:02 to 10:10, suggesting that the subsidence was due to the release of eruption products. Skiers near the gondola station saw a black vertically elongated jet that spouted from the western foot of the Kagamiike-kita Pyroclastic Cone (Sankei News 2018).…”