Long-term seismic background level variations are consistent with volcanic activity at Kusatsu-Shirane volcano, Japan
Abstract
Abstract 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 the SBL increased during the unrest episodes in 2014 and 2018, with a good temporal correspondence with both deep and shallow sources of ground deformation. The SBL increase followed the onset of enhanced seismic activity in 2014, whereas it preceded the seismic activation in 2018. The inter-station SBL ratio also differed between the two unrest periods. One possible interpretation is that the source region of the vibrations contributing to the SBL differed between the 2014 and 2018 unrests. 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 suggest that the SBL may reflect volcanic activity, even without strong seismic or deformation signals. Overall, we infer that the SBL variations over the 20 years were related to volcanic activity, possibly associated with the supply of volcanic fluids.
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