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This study clarifies the hydrothermal system of Jigokudani Valley near Mt. Tateyama volcano in Japan by using a combination of audio-frequency magnetotelluric (AMT) survey and hot-spring water analysis in order to assess the potential of future phreatic eruptions in the area. Repeated phreatic eruptions in the area about 40,000 years ago produced the current valley morphology, which is now an active solfatara field dotted with hot springs and fumaroles indicative of a well-developed hydrothermal system. The three-dimensional (3D) resistivity structure of the hydrothermal system was modeled by using the results of an AMT survey conducted at 25 locations across the valley in 2013–2014. The model suggests the presence of a near-surface highly conductive layer of 2 O isotopic ratios. All hot-spring waters had low pH and could be categorized into three types on the basis of the Cl − /SO 4 2 − concentration ratio, with all falling largely on a mixing line between magmatic fluids and local meteoric water (LMW). The geochemical analysis suggests that the hydrothermal system includes a two-phase zone of vapor–liquid. A comparison of the resistivity structure and the geochemically inferred structure suggests that a hydrothermal reservoir is present at a depth of approximately 500 m, from which hot-spring water differentiates into the three observed types. The two-phase zone appears to be located immediately beneath the cap rock structure. These findings suggest that the hydrothermal system of Jigokudani Valley exhibits a number of factors that could trigger a future phreatic eruption.

The Kusatsu-Shirane volcano is a Quaternary andesitic-to-dacitic active volcano located on the central Honshu Arc. The volcano is known to have had repeated phreatic eruptions in last 200 years. A number of geochemical and geophysical studies have been conducted around the Yugama crater lake, the focus of current volcanic activity. However, the 2018 unexpected phreatic eruption occurred at Mt. Motoshirane, a different pyroclastic cone from that which hosts Yugama. There were also frequent magmatic eruptions until 1500 years ago at this cone and the magma produced at that time has not yet cooled and solidified. A better understanding of the magmatic hydrothermal system of this volcano requires structural information gathered over a larger area, and to greater depths, than has been achieved to date. Here, we describe the three-dimensional (3-D) electrical resistivity structure around Mt. Motoshirane down to a depth of ~10 km using broadband magnetotelluric (MT) data (320–0.0005 Hz) collected in 2015 and 2016. The 3-D resistivity structure model shows an extensive low-resistivity layer at depths of 1–3.5 km beneath the summit. However, structures characteristic of the presence of magma are not observed beneath this layer. It could be too deep or too small to be detected. Combining the new data with results of previous geochemical and geophysical studies, we interpret the conductor as a hydrothermal fluid reservoir that supplies fluids to the crater lake and to hot springs on the eastern and western flanks of the volcano. The existence of a fluid reservoir that extends from the vicinity of the Yugama crater lake to the subsurface beneath an inactive pyroclastic cone that has produced repeated magmatic eruptions in the past suggests that a large-scale magmatic hydrothermal system is developing beneath the volcano.