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Abstract

Geomagnetic field measurements at active volcanoes have been performed to investigate the thermal state within the shallow volcanic edifice. At the Kusatsu-Shirane Volcano, repeated and continuous observations around the Yugama crater lake have been conducted by the Kakioka Magnetic Observatory of the Japan Meteorological Agency since the 1970s. The geomagnetic variations up to 2012 obtained from these observations and their causes were reviewed by Takahashi and Fujii (2014). In this study, we introduce the continuous observations of geomagnetic total intensity conducted since 2010 by the Tokyo Institute of Technology and the observed geomagnetic changes. Since observations began, distinct changes probably attributed to the thermal demagnetization of rocks within the volcanic edifice have been identified twice, associated with the unrest in 2014 and 2018. The features of these variations were summarized, and the locations of the magnetic sources were estimated using simple model calculations. The magnetic dipole source that explains the geomagnetic changes observed during the 2014 unrest was estimated to be deep, which is below the pressure source that explains the tilt change during the same period. However, the dipole source for the geomagnetic changes indicating a demagnetizing trend observed in synchronization with the earthquake swarm activity in April 2018 was found to be located beneath the southeastern rim of the Yugama crater lake, which had never previously been estimated. All these sources are located beneath the highly conductive layer inferred near the surface and are thought to be related to the ascent and accumulation of hydrothermal fluids.