9:00 AM - 10:30 AM
[SVC30-P04] Shallow hydrothermal fluid flow simulations at Kusatsu-Shirane Volcano
Keywords:Kusatsu-Shirane Volcano, numerical simulation, TOUGH3, Hydrothermal system
Phreatic eruptions are one of the eruption styles that are difficult to forecast. Phreatic eruptions are thought to be caused by a rapid expansion of fluids just before the eruption, unlike magmatic eruptions which involve magma; consequently, precursor phenomena are often not observed. In addition, phreatic eruptions often occur at volcanoes with well-developed hydrothermal systems. In such volcanoes, hot springs and fumarolic activity are often utilized as tourism resources, thus, sudden eruptions may occur close to tourist sites. In this study, as a first step to clarify the mechanism of phreatic eruptions, we attempted to reproduce the shallow hydrothermal system of Kusatsu-Shirane Volcano where phreatic eruptions had been repeated.
Kusatsu-Shirane volcano is an active volcano located near the border of Gunma and Nagano prefectures featuring three pyroclastic cone groups; Mt. Shirane hosting the Yugama crater lake, Ainomine, and Mt. Motoshirane, in order from the north. Intensive fumarolic activity and increased seismic activity have been observed around the Yugama crater, and phreatic eruptions have occurred multiple times in the 20th century. In contrast, Mt. Motoshirane has had no substantial surface activity except for an abrupt phreatic eruption in 2018, indicating distinct features from Mt. Shirane.
Many studies have been conducted on Mt. Shirane that showed active surface and subsurface activity. Electromagnetic surveys have revealed the subsurface resistivity structure in the shallow and deep parts, which have been interpreted as the structures of the deep-seated magma and of the cap rocks related to phreatic eruptions (Nurhasan et al., 2006; Matsunaga et al., 2020; Tseng et al, 2021; Matsunaga et al., 2022a). In addition, hydrothermal simulations have been performed for the entire Kusatsu-Shirane Volcano using the permeability structure produced from the inferred resistivity structure (Matsunaga et al., 2022b). However, simulations of shallow hydrothermal systems, where phreatic eruptions occur, have not yet been accomplished. In this study, we aimed to reproduce the shallow hydrothermal system before and after the phreatic eruption, by creating a two-dimensional cylindrical permeability structure using TOUGH3.
Kusatsu-Shirane volcano is an active volcano located near the border of Gunma and Nagano prefectures featuring three pyroclastic cone groups; Mt. Shirane hosting the Yugama crater lake, Ainomine, and Mt. Motoshirane, in order from the north. Intensive fumarolic activity and increased seismic activity have been observed around the Yugama crater, and phreatic eruptions have occurred multiple times in the 20th century. In contrast, Mt. Motoshirane has had no substantial surface activity except for an abrupt phreatic eruption in 2018, indicating distinct features from Mt. Shirane.
Many studies have been conducted on Mt. Shirane that showed active surface and subsurface activity. Electromagnetic surveys have revealed the subsurface resistivity structure in the shallow and deep parts, which have been interpreted as the structures of the deep-seated magma and of the cap rocks related to phreatic eruptions (Nurhasan et al., 2006; Matsunaga et al., 2020; Tseng et al, 2021; Matsunaga et al., 2022a). In addition, hydrothermal simulations have been performed for the entire Kusatsu-Shirane Volcano using the permeability structure produced from the inferred resistivity structure (Matsunaga et al., 2022b). However, simulations of shallow hydrothermal systems, where phreatic eruptions occur, have not yet been accomplished. In this study, we aimed to reproduce the shallow hydrothermal system before and after the phreatic eruption, by creating a two-dimensional cylindrical permeability structure using TOUGH3.