[S-VC35] Hydrothermal systems of volcanoes

[SVC35-04] Electromagnetic monitoring survey in Kusatsu-Shirane Volcano, Japan

*Keiichi Ishizu¹, Yasuo Ogawa², Norihiro Kitaoka², Kuo-Hsuan Tseng², Sohei Serita², Takuto Minami³, Takahiro Kunitomo⁴, Hiroshi Ichihara⁵, Grant Caldwell⁶ (1.Kyushu University, 2.Institute of Science Tokyo, 3.Kobe University, 4.Ontake Science Lab, 5.Nagoya University, 6.GNS Science)

Keywords:Volcano, CSEM, Time variation

At Kusatsu-Shirane Volcano, where phreatic eruptions have repeatedly occurred, we have conducted electromagnetic (EM) monitoring to investigate temporal variations in the hydrothermal system related to these eruptions. This involves capturing the subsurface resistivity structure and its changes to infer hydrothermal system dynamics. To ensure continuous data acquisition with consistent accuracy, we use a noise-resistant system employing a controlled-source EM transmitter. Current is transmitted from the Ishizu sulfur mine, 4 km from the Yugama crater, and the response is measured by eight receivers deployed around the crater. Data has been collected from October 2020 to October 2024, excluding snowy periods. An initial resistivity model was derived from October 2020 data. Notably, this model revealed the spatial distribution of steam-dominated hydrothermal fluids, which was previously undetected by magnetotellurics surveys. Time-series analysis of data from 2020 to 2021 was performed to estimate resistivity changes from the 2020 model. Comparison of EM responses calculated from October 2020 and May 2021 data shows a phase change of several degrees, suggesting subsurface resistivity variations.