11:15 AM - 11:30 AM
[SVC28-08] Temporal Variation in Volcanic Gas Composition at Hakone Volcano, Japan
Keywords:Volcanic gas, Fumarolic gas, Mt Hakone
Preface) A phreatic eruption is a rupture of the earth's crust due to an increase in the pressure of a hydrothermal reservoir, and a precursor to an eruption may appear as a change in the state of the hydrothermal reservoir. A portion of the gas phase present in hydrothermal reservoir is discharged to the surface as volcanic gas (fumarolic gas). Two months before the phreatic eruption of Mt. Hakone in 2015, changes in the chemical composition of fumarolic gas was observed. The observation of fumarolic gas is thought to contribute to understanding the precursors of phreatic eruptions. After a phreatic eruption in 2015, Mt. Hakone experienced frequent volcanic earthquakes in 2019, and the eruption alert level was raised to 2. In this study, we will focus on the volcanic gas discharging from Mt. Hakone and consider the temporal changes in its chemical composition.
Collection and analysis of volcanic gas) Fumarolic gas is repeatedly collected once a month at fumaroles N and C in the Owakudani geothermal area of Mt. Hakone, and fumarole S in the Kamiyuba geothermal area located approximately 700 m north of Owakudani geothermal area. Observations began in 2013 at fumaroles N and S. Observations at Fumarole C began in 2019. Wet analysis and gas chromatography were used to analyze the collected fumarolic gas.
Results/discussion) Among the three fumaroles (N, S, C), the fumarolic gas C contains a relatively high concentration of SO2, and its chemical composition is similar to the gas released from the adjacent 500 m deep borehole. . The chemical composition of fumarolic gas C is thought to be representative of the composition of volcanic gas approximately 500 meters underground. Furthermore, changes in the chemical composition of fumarolic gas C have a higher correlation with changes in the number of earthquakes than fumarolic gases N and S. From July to August 2021, the He/CH4 ratio of fumarolic gas C increased rapidly, and then continued a gradual decreasing trend until April 2023. From April to May 2023, the He/CH4 ratio increases rapidly again and remains at a high value until January 2024. One year and nine months have passed from July 2021 to April 2023, which is in line with the empirical rule that volcanic activity of Mt Hakone increases every two years. We define here Period 1 (January to April 2023) to be the three months before the He/CH4 ratio increases, and Period 2 (May to August 2023) to be the three months after the He/CH4 ratio increases. The average values of Log(He/CH4) in Periods 1 and 2 were -1.5 and -0.5, respectively. In other words, the He/CH4 ratio increased 10 times throughout Periods 1 and 2. The average values of Log(He/H2O) in Periods 1 and 2 were -7.5 and -7.6, respectively. In other words, the He/H2O ratio hardly changes throughout Periods 1 and 2. The average values of Log(CH4/H2O) in Periods 1 and 2 were -6.0 and -7.1, respectively. In other words, the CH4/H2O ratio decreased to one-tenth throughout Periods 1 and 2. The increase in the He/CH4 ratio that occurred from April to May 2023 can be said to be caused by the decrease in the CH4/H2O ratio. CH4 is a component derived from hydrothermal systems or the sediment of crust, and is hardly included in fluids originating from magma. CH4 is thought to flow into the area approximately 500 meters underground where the gas from fumarole c is contained. It is thought that the increase or decrease in the flux of CH4 controls the He/CH4 ratio of fumarolic gas C.
Collection and analysis of volcanic gas) Fumarolic gas is repeatedly collected once a month at fumaroles N and C in the Owakudani geothermal area of Mt. Hakone, and fumarole S in the Kamiyuba geothermal area located approximately 700 m north of Owakudani geothermal area. Observations began in 2013 at fumaroles N and S. Observations at Fumarole C began in 2019. Wet analysis and gas chromatography were used to analyze the collected fumarolic gas.
Results/discussion) Among the three fumaroles (N, S, C), the fumarolic gas C contains a relatively high concentration of SO2, and its chemical composition is similar to the gas released from the adjacent 500 m deep borehole. . The chemical composition of fumarolic gas C is thought to be representative of the composition of volcanic gas approximately 500 meters underground. Furthermore, changes in the chemical composition of fumarolic gas C have a higher correlation with changes in the number of earthquakes than fumarolic gases N and S. From July to August 2021, the He/CH4 ratio of fumarolic gas C increased rapidly, and then continued a gradual decreasing trend until April 2023. From April to May 2023, the He/CH4 ratio increases rapidly again and remains at a high value until January 2024. One year and nine months have passed from July 2021 to April 2023, which is in line with the empirical rule that volcanic activity of Mt Hakone increases every two years. We define here Period 1 (January to April 2023) to be the three months before the He/CH4 ratio increases, and Period 2 (May to August 2023) to be the three months after the He/CH4 ratio increases. The average values of Log(He/CH4) in Periods 1 and 2 were -1.5 and -0.5, respectively. In other words, the He/CH4 ratio increased 10 times throughout Periods 1 and 2. The average values of Log(He/H2O) in Periods 1 and 2 were -7.5 and -7.6, respectively. In other words, the He/H2O ratio hardly changes throughout Periods 1 and 2. The average values of Log(CH4/H2O) in Periods 1 and 2 were -6.0 and -7.1, respectively. In other words, the CH4/H2O ratio decreased to one-tenth throughout Periods 1 and 2. The increase in the He/CH4 ratio that occurred from April to May 2023 can be said to be caused by the decrease in the CH4/H2O ratio. CH4 is a component derived from hydrothermal systems or the sediment of crust, and is hardly included in fluids originating from magma. CH4 is thought to flow into the area approximately 500 meters underground where the gas from fumarole c is contained. It is thought that the increase or decrease in the flux of CH4 controls the He/CH4 ratio of fumarolic gas C.