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 and phreatic eruptions. On 23 January 2018, a phreatic eruption occurred at the Motoshirane cone of Kusatsu-Shirane, where no considerable volcanic activity had been reported in observational and historical records. To understand the eruption process of this unique event, we analyzed seismic, tilt, and infrasound records. The onset of surface activity accompanied by infrasound signal was preceded by volcanic tremor and inflation of the volcano for ~ 2 min. Tremor signals with a frequency band of 5–20 Hz remarkably coincide with the rapid inflation. We apply an amplitude source location method to seismic signals in the 5–20 Hz band to estimate tremor source locations. Our analysis locates tremor sources at 1 km north of Motoshirane and at a depth of 0.5–1 km from the surface. Inferred source locations correspond to a conductive layer of impermeable cap-rock estimated by magnetotelluric investigations. An upper portion of the seismogenic region suggests hydrothermal activity hosted beneath the cap-rock. Examined seismic signals in the 5–20 Hz band are typically excited by volcano-tectonic events with faulting mechanism. Based on the above characteristics and background, we interpret that excitation of examined volcanic tremor reflects small shear fractures induced by sudden hydrothermal fluid injection to the cap-rock layer. The horizontal distance of 1 km between inferred tremor sources and Motoshirane implies lateral migration of the hydrothermal fluid, although direct evidence is not available. Kusatsu-Shirane has exhibited unrest at the Yugama lake since 2014. However, the inferred tremor source locations do not overlap active seismicity beneath Yugama. Therefore, our result suggests that the 2018 eruption was triggered by hydrothermal fluid injection through a different pathway from that has driven unrest activities at Yugama.
Highlights
Kusatsu-Shirane volcano consists of three pyroclastic cones (Shirane, Ainomine, and Motoshirane), central Japan (Fig. 1)
Inferred source locations correspond to an impermeable cap-rock layer and an upper portion of the seismogenic region beneath the Ainomine cone
Seismic signals in the 5–20 Hz band correspond to dominant signals accompanying Volcano Tectonic (VT) events with faulting mechanism
Summary
Kusatsu-Shirane volcano consists of three pyroclastic cones (Shirane, Ainomine, and Motoshirane), central Japan (Fig. 1). We apply the ASL technique to seismic records accompanying the 2018 eruption to estimate volcanic tremor source locations. We adopt seismic records at KSE, KSS, KSW, KASO, and KSHA in the ASL based on the observed SNR, distances from the Motoshirane cone, and operational history, including the 2011 unrest. Our reference time window is set from 10:02:10, including the origin time of dominant infrasound signals, and with source location at the surface of the center of active vents (Fig. 2a). Following Ichihara et al (2012), this pressure change can induce ground velocity on the order of 10–7 m/s, which is one order of magnitude smaller than the observed tremor amplitude in the reference time window (Fig. 3). We focus on the time window for 210 s from 09:59:00
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