ALBERT

Description: Phreatic eruptions are one of the eruption styles that are difficult to forecast and occasionally cause significant damage. Understanding the mechanisms of phreatic eruption is essential for minimizing damage. Mt. Motoshirane, one of the pyroclastic cones of the Kusatsu-Shirane Volcano located in central Japan, experienced a phreatic eruption in 2018 that resulted in casualties. The eruption was abrupt with no precursory phenomena. Mt. Motoshirane has been dormant for the last 1500 years, so it has been poorly monitored and studied. Studies conducted after the eruption suggested that the migration of hydrothermal fluids triggered the eruption, but those studies lacked information on shallow subsurface structure. In this paper, we report on the three-dimensional resistivity structure around the craters of the 2018 eruption, which was estimated from the audio-frequency magnetotelluric data obtained in 2020 and 2022. The inferred resistivity structure basically has a two-layer structure composed of a high-resistivity layer corresponding to the Quaternary lavas near the surface and low resistivities corresponding to the altered Neogene lavas below. Shallow moderately high resistivity regions in the low-resistivity layer are considered fluid reservoirs that have become infiltrated as a result of the eruption. Regions around the crack associated with the eruption showed relatively high resistivity, implying that the low resistivity zone is decoupled beneath the eruption site. We will discuss how the phreatic eruption occurred based on these interpretations in the presentation.