Volcanic unrest at Hakone volcano after the 2015 phreatic eruption: reactivation of a ruptured hydrothermal system?

Abstract Since the beginning of twenty-first century, volcanic unrest has occurred every 2–5 years at Hakone volcano. After 2015 eruption, activity changed significantly in terms seismicity and geochemistry. Like pre- co-eruptive unrest, each post-eruptive episode was detected by deep inflation below volcano (~ 10 km) low frequency events, which can be interpreted as reflecting supply magma or magmatic fluid from depth. The seismic during episodes also increased; however, beneath eruption center lower, especially shallow region 2 km), while sporadic swarms were observed caldera rim, ~ 3 km away center. This observation a recent InSAR analysis imply that hydrothermal system could composed multiple sub-systems, host earthquake show independent volume changes. established routes for steam sub-system (≥ 150 m deep) to surface through cap-rock, allowing emission super-heated 160 ºC). showed an increase magmatic/hydrothermal gas ratios (SO /H S HCl/H S) 2019 episode; no indicated geodetic observations. Net SO episodes, remained within usual range period, is inconsistent with intrusion. We consider triggered newly derived depth; breached cap-rock unable allow subsequent pressurization intensive heat released dried vapor-dominated portion inhibited scrubbing HCl higher ratio. have sealing zone near brittle–ductile transition self-sealing process seems not completed based on observations episodes.