Long-term seismic background level variations reveal volcanic fluid supply and serve as a sensitive indicator for volcano monitoring at Kusatsu-Shirane volcano, Japan

Kusatsu-Shirane volcano in central Japan has repeatedly experienced phreatic eruptions, with the Yugama crater being one of the main centers of activity. To clarify long-term variations in volcanic activity around the Yugama crater, we analyzed more than 20 years of continuous seismic records using a statistical approach to calculate the Seismic background level (SBL). This method enables the detection of weak volcanic signals that are buried in background noise. The results revealed that SBL increased during the unrest episodes in 2014 and 2018, with a strong correlation with both deep and shallow sources of ground deformation. These SBL increases are interpreted as reflecting changes in volcanic activity associated with the supply of volcanic fluids from deeper to shallower regions within the volcanic system. The SBL ratio between stations also changed during these unrest periods, indicating differences in the spatial distribution of the SBL vibration source. Variations in SBL were also detected during non-unrest periods, coinciding with minor surface phenomena such as color changes of the Yugama crater lake and the appearance of new fumaroles. These correspondences imply that the SBL reflects even subtle fluctuations in volcanic and hydrothermal activity without strong seismic or deformation signals. Overall, this study demonstrates that SBL analysis provides a sensitive and quantitative means of detecting weak volcanic signals associated with subsurface fluid activity. Although SBL analysis alone cannot replace conventional monitoring techniques, it offers a powerful tool for detecting subtle variations in volcanic activity. When combined with SBL ratio and particle motion analyses, it also provides spatial information about the vibration source, thereby improving volcano monitoring in hydrothermal and phreatic-eruption environments.