14:30 〜 14:45
[SVC28-04] Water content in phreatic jet at Ioto volcano: Integration of geological and geophysical approaches
キーワード：水蒸気噴火、アイソパック、インフラサウンド、熱水、硫黄島火山
Phreatic eruptions are ubiquitously observed in active volcanoes, and sourced from solid (country rock, altered rock, mud, and etc)-water system underground. It is surely that the driving force of the eruptions is expansion of water in the system, however, evaluation of water content of the system driving phreatic eruption has not been enough. The quantitative evaluation of the water content is essential for understandings on the mechanism of the phreatic eruptions.
In this study, we estimate the water content in solid-water system driving the phreatic eruptions, by combining the geological and geophysical approaches. We dealt with three phreatic eruptions (Feb 7-9, 2012; Feb 17-18, 2013; Apr 11, 2013) at Ioto volcano, Japan. The geological survey few months after the occurrence gives eruptive volume of the three phreatic eruptions (0.8×103 m3 for Feb 7-9, 2012; 1.1×104 m3 for Feb 17-18, 2013; 2.7×103 m3 for Apr 11, 2013). On the other hand, the seismic and infrasound analyses give the total volume emitted (solid+liquid+gas) in the three eruptions (5.0×105 m3 for Feb 7-9, 2012; 1.8×107 m3 for Feb 17-18, 2013; 1.6×106 m3 for Apr 11, 2013). Subtracting the solid volume from the total volume provides a volume of water emitted. The water volume was converted to the mass by using ideal gas equation of state (PV=nRT) and temperature of 200±100 degC. Then, the mass fractions of water in the three phreatic eruptions were estimated to 13-33 wt.%.
The result of our study shows that there are positive correlations among the masses of water and solid, and mass fraction of water. This fact suggests that large hydrothermal chamber generates large scale phreatic eruption. High water flux from deeper part can generate large hydrothermal chamber, fueling larger eruptive event.
In this study, we estimate the water content in solid-water system driving the phreatic eruptions, by combining the geological and geophysical approaches. We dealt with three phreatic eruptions (Feb 7-9, 2012; Feb 17-18, 2013; Apr 11, 2013) at Ioto volcano, Japan. The geological survey few months after the occurrence gives eruptive volume of the three phreatic eruptions (0.8×103 m3 for Feb 7-9, 2012; 1.1×104 m3 for Feb 17-18, 2013; 2.7×103 m3 for Apr 11, 2013). On the other hand, the seismic and infrasound analyses give the total volume emitted (solid+liquid+gas) in the three eruptions (5.0×105 m3 for Feb 7-9, 2012; 1.8×107 m3 for Feb 17-18, 2013; 1.6×106 m3 for Apr 11, 2013). Subtracting the solid volume from the total volume provides a volume of water emitted. The water volume was converted to the mass by using ideal gas equation of state (PV=nRT) and temperature of 200±100 degC. Then, the mass fractions of water in the three phreatic eruptions were estimated to 13-33 wt.%.
The result of our study shows that there are positive correlations among the masses of water and solid, and mass fraction of water. This fact suggests that large hydrothermal chamber generates large scale phreatic eruption. High water flux from deeper part can generate large hydrothermal chamber, fueling larger eruptive event.