Volcanic Unrest at Hakone Volcano after the 2015 phreatic eruption — Reactivation of a Ruptured Hydrothermal System?

Abstract

Since the beginning of the 21st century, volcanic unrest has occurred every 2–5 years at Hakone volcano. After the 2015 eruption, unrest activity changed significantly in terms of seismicity and geochemistry. Like the pre- and co-eruptive unrest, each post-eruptive unrest episode was detected by deep inflation below the volcano (~ 10 km) and deep low frequency events, which can be interpreted as reflecting supply of magma or magmatic fluid from depth. The seismic activity during the post-eruptive unrest episodes also increased; however, seismic activity beneath the eruption center during the unrest episodes was significantly lower, especially in the shallow region (~2 km), while sporadic seismic swarms were observed beneath the caldera rim, ~3 km away from the center. This observation and a recent InSAR analysis imply that the hydrothermal system of the volcano could be composed of multiple sub-systems, each of which can host earthquake swarm and show independent volume change. The 2015 eruption established routes for steam from the hydrothermal sub-system beneath the eruption center (≥ 150 m deep) to the surface through the cap-rock, allowing emission of super-heated steam (~ 160 ºC). This steam showed an increase in magmatic/hydrothermal gas ratios (SO2/H2S and HCl/H2S) in the 2019 unrest episode; however, no magma supply was indicated by seismic and geodetic observations. Net SO2 emission during the post-eruptive unrest episodes, which remained within the usual range of the post-eruptive period, is also inconsistent with shallow intrusion. We consider that the post-eruptive unrest episodes were also triggered by newly derived magma or magmatic fluid from depth; however, the breached cap-rock was unable to allow subsequent pressurization and intensive seismic activity within the hydrothermal sub-system beneath the eruption center. The heat released from the newly derived magma or fluid dried the vapor-dominated portion of the hydrothermal system and inhibited scrubbing of SO2 and HCl to allow a higher magmatic/hydrothermal gas ratio. The 2015 eruption could have also breached the sealing zone near the brittle–plastic transition and the subsequent self-sealing process seems not to have completed based on the observations during the post-eruptive unrest episodes.

Hydrothermal and Magmatic System of a Volcanic Island Inferred From Magnetotellurics, Seismicity, Self‐potential, and Thermal Image: An Example of Miyakejima (Japan)

“…Hence, the central (and most conductive) domain of the clay cap may act as a low‐permeable boundary limiting fluid circulation. The local rupture of this layer, (due to a dike intrusion or the pressurization of underlying fluids) can transiently release a large amount of heat in the shallow hydrothermal system, which can generate a sudden explosive eruption (e.g., Mannen et al., 2021).…”

Observing Posteruptive Deflation of Hydrothermal System Using InSAR Time Series Analysis: An Application of ALOS‐2/PALSAR‐2 Data on the 2015 Phreatic Eruption of Hakone Volcano, Japan

“…These lines of evidence indicate that the phreatic eruption was triggered by hydrothermal fluids stored approximately 225 m above sea level, which migrated toward the shallower part of the edifice through the crack during the eruption. Since the 2015 phreatic eruption, fumarolic activity in Owakudani has been higher than before (Mannen et al., 2021). This higher steam activity during and after the eruption suggests the rupturing of the sealed and pressured hydrothermal system beneath the volcano during the 2015 eruption, as indicated by general modeling of hydrothermal systems (e.g., Fournier, 1999; Stix & de Moor, 2018).…”