2021 年 130 巻 6 号 p. 783-796
Earthquake swarms have occurred with volcanism repeatedly at Hakone volcano in Kanagawa prefecture, Japan. In 2015, a phreatic eruption took place about two months after the start of an earthquake swarm. Hakone volcano is a popular tourist destination. If it is possible to forecast at the early stages of an earthquake swarm whether or not an eruption will occur, the forecast could contribute to preventing disasters involving tourists. At Hakone volcano, increases in the ratio of components (CO2/H2S) contained in the volcanic gas from fumaroles were observed in synchronization with earthquake swarms and ground deformation in 2013 and 2015. Similar increases in CO2/H2S ratio were also observed in 2017 and 2019, although the increases in the CO2/H2S ratio in 2017 and 2019 were not as sharp as those in 2013 and 2015. Furthermore, the maximum values of the CO2/H2S ratio in 2017 and 2019 were lower than the values in 2013 and 2015. These differences in the CO2/H2S ratio may be related to the limited and smaller scale of volcanic activity in 2017 and 2019 relative to 2013 and 2015. Another explanation for the difference is a possible irreversible change in the underground structure of the Owakudani area, which is a geothermal area around Hakone volcano, because the phreatic eruption took place in the Owakudani area in 2015. During all four seismically active periods in 2013, 2015, 2017, and 2019, the CO2/H2S ratio decreased simultaneously with decreases in the number of volcanic earthquakes. The lower limit of CO2/H2S ratios after the peak of the CO2/H2S ratio time series was about 20 in all periods. This implies that subsequent unrest would not start until the CO2/H2S ratio drops to about 20. The CO2/H2S ratio might be an effective tool for forecasting activity at Hakone volcano. During the active periods in 2013, 2015, 2017, and 2019, extensions of the baseline across Hakone volcano were observed by GNSS with increases in the CO2/H2S ratio. A good correlation was found between the extensional velocity of the baseline length and the increasing rate of the CO2/H2S ratio. These variations could be examined at the early stage of unrest. The findings argue that the CO2/H2S ratio is a promising tool for predicting and evaluating volcanic activity at Hakone volcano.