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.

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.

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