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Fig. 12 | Earth, Planets and Space

Fig. 12

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

Fig. 12

Schematic model of the pre- and post-eruptive hydrothermal systems. Not drawn to scale. See text for depth and dimension of the features. a) Before the 2015 eruption, cap-rock and sealing at the brittle–ductile transition (BDT) were intact and magmatic gas accumulated within the pressurized pores beneath the BDT. We assume individual hydrothermal systems beneath the central cones and caldera rim are separated by a low-permeable sealing. b) Ruptures of sealing at the BDT induced by a deep inflation event allowed rapid migration of accumulated heat, fluid and pore pressure beneath the BDT to the liquid-dominated zones of the hydrothermal systems. Volcano-tectonic earthquakes (VTs) and steaming from the steam production wells (SPW) increased due to the pressurization of the hydrothermal system. c) The 2015 eruption formed paths for steam from the vapor-dominated (VD) zone to the surface. Due to the path, the post-eruptive hydrothermal system beneath the central cone was depressurized and volume of the VD zone increased. The rupture of seals at the BDT healed beneath the caldera rim, but were maintained beneath the central cones, which allows migration of heat and pore fluid beneath the BDT. d) Post-eruptive unrest episodes were also triggered by deep inflation event. Beneath the central cones, no extensive pressurization of the hydrothermal system occurs because pores beneath the BDT were not pressurized due to the rupturing in the 2015 eruption. Heat migration from deep to shallow lowered the liquid phase fraction within the VD zone and caused less effective scrubbing, which allowed enrichment of soluble gas (SO2 and HCl) within the steams from the VD zone and fumaroles. Magmatic CO2 within the fumaroles also rose, but not significantly due to no release of accumulated CO2 within the pressurized pores beneath the BDT

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