Locating hydrothermal fluid injection of the 2018 phreatic eruption at Kusatsu-Shirane volcano with volcanic tremor amplitude

Abstract Kusatsu-Shirane volcano hosts numerous thermal springs, fumaroles, and the crater lake of Yugama. Hence, it has been a particular study field for hydrothermal systems phreatic eruptions. On 23 January 2018, eruption occurred at Motoshirane cone Kusatsu-Shirane, where no considerable volcanic activity had reported in observational historical records. To understand process this unique event, we analyzed seismic, tilt, infrasound The onset surface accompanied by signal was preceded tremor inflation ~ 2 min. Tremor signals with frequency band 5–20 Hz remarkably coincide rapid inflation. We apply an amplitude source location method to seismic estimate locations. Our analysis locates sources 1 km north depth 0.5–1 from surface. Inferred locations correspond conductive layer impermeable cap-rock estimated magnetotelluric investigations. An upper portion seismogenic region suggests hosted beneath cap-rock. Examined are typically excited volcano-tectonic events faulting mechanism. Based on above characteristics background, interpret that excitation examined reflects small shear fractures induced sudden fluid injection layer. horizontal distance between inferred implies lateral migration fluid, although direct evidence is not available. exhibited unrest Yugama since 2014. However, do overlap active seismicity Therefore, our result 2018 triggered through different pathway driven activities