Time variations in the chemical and isotopic composition of fumarolic gases at Hakone volcano, Honshu Island, Japan, over the earthquake swarm and eruption in 2015, interpreted by magma sealing model

Abstract

Definite increases in the components ratios of CO 2 /H 2 O, CO 2 /H 2 S, CO 2 /CH 4 and He/CH 4 were observed at the fumarolic gases from Owakudani geothermal area located at the center of Hakone volcanic caldera (Honshu Island, Japan), synchronized with the earthquake swarm in 2015. Such variations were due to the dominance of a magmatic component over a hydrothermal component, suggesting the earthquake swarm was produced by the injection of magmatic gases into the hydrothermal system. The CO 2 /H 2 O ratio of magmatic gas was estimated to be 0.0045 before the earthquake swarm, which increased up to 0.013 during the earthquake swarm, likely produced by the pressurization of magma as a result of magma sealing where the pressure increment in magma was estimated to be 3% to the lithostatic pressure. The H 2 O and CO 2 concentration in magma were estimated to be 6.3 wt% and 20 wt ppm, respectively, assuming a temperature 900 °C and a rhyolitic composition. In May 2015, a few months prior to the earthquake swarm in May 2015, a sharp increase in the Ar/CO 2 and N 2 /He ratios and a decrease in the isotopic ratio of H 2 O were observed at the fumarolic gas. The invasion of air into the hydrothermal system increased the Ar/CO 2 and N 2 /He ratios. The decrease in the isotopic ratio of H 2 O was induced by partial condensation of H 2 O vapor.

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

“…The onset of the 2015 unrest was rst recognized in early April from increases in DLFs and the baseline length across the volcano detected by GNSS, which were interpreted as in ation of magma chamber due to addition of magma or magmatic uid (Harada et al 2018;Mannen et al 2018;Yukutake et al 2019) (Figure 3). Then an earthquake swarm, a blowout of SPW39, and an increase in the CO 2 /H 2 O ratio of the fumarole gas emitted near the future eruption center followed (Mannen et al 2018;Ohba et al 2019). Although the seismicity and the CO 2 /H 2 O ratio began decreasing after mid-May, a small phreatic eruption occurred on the morning of June 29 and lasted until the early morning of July 1 (Yukutake et al 2017;Mannen et al 2018).…”

“…Geochemical monitoring has provided evidence for development of a sealing zone ( Figure 2) and injection of magmatic uid into the hydrothermal system through the zone (Ohba et al 2019). Very shallow geological and resistivity structures (≤ 500 m deep) are summarized in Mannen et al (2019); the very shallow in ation source of the 2015 eruption (Doke et al 2018;Kobayashi et al 2018), which was interpreted as a vapor pocket located 150 m deep beneath the eruption center ( Figure 2; surface elevation is approximately 1000 m above sea level) was determined by a high-resolution magnetotelluric survey (CSAMT) to be a high resistivity zone within the apex of the bell-shaped conductive body (Yoshimura et al 2018).…”

Special issue “Towards forecasting phreatic eruptions: examples from Hakone volcano and some global equivalents”

“…To understand phreatic eruptions, the characterization of the properties of these structures, including their location, depth, and function in the context of the magma-hydrothermal system of the volcano is necessary. In the special issue, three contributions focused on investigations of cap rock and sealing zones (Ueda et al 2018;Yoshimura et al 2018;Ohba et al 2019). Yoshimura et al (2018) conducted an audio-frequency magnetotelluric (AMT) survey at 39 sites, covering the whole of Hakone caldera before the eruption, and established a three-dimensional model of the resistivity structure of the volcano.…”

“…Since the previous study proposed that the seismic activity of the volcano had been triggered by pressure rise or fluid migration (Yukutake et al 2011), the bell-shaped conductor is interpreted as the cap rock that confines a hydrothermal system. Ohba et al (2019) compiled long-term gas observations and developed a hydrothermal model of Hakone volcano in which a sealing zone just above the magma chamber controls volcanic unrest. In the model, the sealing zone has some permeability during background periods; however, a few months before the onset of volcanic unrest, changes in permeability of the zone began to restrict gas migration from the magma chamber to the hydrothermal system.…”

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

“…Geochemical monitoring has provided evidence for development of a sealing zone and injection of magmatic uid into the hydrothermal system through the zone(Ohba et al 2019).Very shallow geological and resistivity structures (≤ 500 m deep) are summarized in Mannen et al (2019); the very shallow in ation source of the 2015 eruption (Doke et al 2018; Kobayashi et al 2018), which was interpreted as a vapor pocket located 150 m deep beneath the eruption center (surface elevation is approximately 1000 m above sea level) was determined by a high-resolution magnetotelluric survey (CSAMT) as a high resistivity zone within the apex of the bell-shaped conductive body (Yoshimura et al 2018). The 2015 eruption and unrest The time sequence of the 2015 unrest and eruption of Hakone volcano was already summarized in Mannen et al (2018).…”