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 2018 phreatic eruption of Mt. Motoshirane on Kusatsu–Shirane volcano, Japan, based data obtained from a network eight tiltmeters weather radar echoes. The tilt records can be explained by subvertical crack model. Small craters that were formed previous eruptions are aligned WNW–ESE, which is consistent with strike modeled in this study. direction maximum compressive stress region horizontal oriented allowing fluid to intrude depth through orientation. Based model, hypocenter distribution, MT resistivity structure, we infer hydrothermal reservoir at 2 km below volcano has repeatedly ascended pre-existing crack. inflation deflation volumes during estimated have been 5.1 × 10 5 3.6 m 3 , respectively, meaning 1.5 expanded volume underground. total heat associated ≥ 14 J, similar or exceeding annual released Yugama Crater Lake Shirane largest past 130 year. Although ejecta was small, not negligible terms budget volcano. A water 0.1–2.0 7 kg discharged as volcanic cloud, echoes, smaller than deflation. suggest underground acted buffer against sudden intrusion fluids, absorbing some