Time Variation in the Chemical and Isotopic Composition of Fumarolic Gasses at Kusatsu-Shirane Volcano, Japan

Abstract

Minor seismicity may occur at volcanoes with hydrothermal system before a steam eruption. To forecast any steam eruption, it is indispensable to detect and understand the nature of this shallow seismicity. As the fumarolic gas resides in the hydrothermal system, it may provide insights for elucidating the nature of any seismicity and thus forecast steam eruptions. At Kusatsu-Shirane volcano Japan, intense seismic activity took place in 2014 and 2018. To investigate the relationship between the seismicity and gas chemistry, five fumarolic gas discharges have been repeatedly analyzed. Since July 2014 to November 2017 a monotonic decrease in CO 2 /H 2 O, He/H 2 O and N 2 /H 2 O ratios was recorded in the fumarolic gasses located north of the summit of volcano, suggesting the decline of the magmatic component. On the contrary the CH 4 /H 2 O ratio significantly increased during the seismically quiet period, indicating that reduced conditions developed in the hydrothermal system, favoring the formation of CH 4. The high N 2 /He ratio in the quiet period indicates the addition of N 2 , likely deriving from the crustal rocks hosting hydrothermal reservoir. The N 2 /He ratio in 2018 was significantly lower than those recorded in 2014, indicating the evolution of magma with the progress of degassing. The δD(H 2 O) and δ 18 O(H 2 O) values and the CO 2 /H 2 O ratios of fumarolic gas discharges were modeled with the following processes: generation of vapor phase after the mixing between magmatic gas and a cold groundwater with meteoric origin, addition of vapor phase with meteoric origin, and partial condensation of water vapor near surface. Only a single magmatic gas is necessary for the above modeling. These data suggest that at Kusatsu-Shirane volcano the activation of seismicity was synchronized with the increase of the magmatic component in the fumarolic gas. It is postulated that the injection of magmatic gas increased the fluid pressure in the reservoir, which triggered seismicity. The injection would have been triggered by a break of the sealing zone surrounding the degassing magma. The injection of magmatic gas can be detected by monitoring the composition of the fumarolic gas, thus giving the possibility to forecast any future seismicity.

Anatomy of active volcanic edifice at the Kusatsu–Shirane volcano, Japan, by magnetotellurics: hydrothermal implications for volcanic unrests

“…The hydrothermal system of the Kusatsu-Shirane volcano has been intensively studied by performing geochemical analysis at the Yugama crater lake and the surrounding fumaroles and hot springs (Sano et al 1994;Ohba et al 2019). Ohba et al (2019) showed a model of a two-phase gas-liquid reservoir underlain by a magmatic fluid reservoir based on the isotope ration of hydrogen and oxygen in relation to recent volcanic unrest.…”

The 2018 phreatic eruption at Mt. Motoshirane of Kusatsu–Shirane volcano, Japan: Eruption and intrusion of hydrothermal fluid observed by a borehole tiltmeter network

“…The latest eruption of the SPCG occurred at YCL in 1982-1983. Recently, microearthquake swarms accompanied by ground deformation and changes in the chemistry of YCL and nearby fumaroles were detected in 1991-1992(Ohba et al 2008, 2019b.…”

“…From March to 2016, the SPCG experienced a microearthquake swarm, ground deformation indicating an increase in volume of 1.2 × 10 5 m 3 over 20 months (Fig. S2), an increase in the Cl and SO4 concentration of YCL, and an increase in the CO2/H2S ratio of nearby fumaroles (SPCG unrest in 2014) (Ohba et al 2019b). Moreover, three months after the 2018 MPCG eruption, a shallow inflation and a marked increase in seismicity occurred at SPCG from April 2018 (MPCG-SPCG unrest in 2018).…”

Groundwater Interacting at Depth With Hot Plastic Magma Triggers Phreatic Eruptions at Yugama Crater Lake of Kusatsu-Shirane Volcano (Japan)

“…As expected, earthquake swarms reoccurred around the Yugama crater from March 2014 and in April 2018. Ohba et al (2019b) estimated that the earthquake swarms in 2014 and 2018 were triggered by the increased pressure in the hydrothermal reservoir due to the injection of magmatic gas, a model based on the analytical results of the chemical and stable isotopic compositions of the fumarolic gases near the Yugama crater. This result made us expect that magma-hydrothermal interaction can be inferred through the chemical composition of Yugama Crater Lake because magmatic gas contains many watersoluble components.…”