[S-VC33] Hydrothermal systems of volcanoes

[SVC33-04] Magma-hydrothermal system at Yakedake Volcano, Japan, inferred from fumarolic gases and self-potential observations

*Takeshi Saito¹, Tensei Hadano², Shinji Ohsawa³, Taketoshi Mishima³, Kazuhiro Amita⁴ (1.Institute of Science, Academic Assembly School of Science and Technology, Shinshu University, 2.Graduate School of Science and Technology, Shinshu University, 3.Institute for Geothermal Sciences, Kyoto University, 4.Graduate School of Engineering Science, Akita University)

Keywords:phreatic eruption, fumarolic gas, self-potential, hydrothermal system

Yakedake volcano had produced numerous phreatic eruptions during the past hundred years. Since the latest phreatic eruption in 1962-1963, Yakedake volcano has remained dormant, but fumarolic activity around the summit area has continued steadily, suggesting the volcanic hydrothermal system has developed under the volcano. In this study, fumarolic gases were observed in order to investigate the gas supply systems. Self-potentials (SP) surveys were conducted in order to examine subsurface hydrothermal flows.
The chemical and isotopic compositions of fumarolic gases have been stable for the past six years, compared to the values obtained shortly after the latest eruption. While the fumarolic gases from the northern summit showed strong influence of magmatic fluid, the gases from the 1962-63 fumaroles suggested subsurface volcanic hydrothermal system under the vent.
As a result of SP surveys, large scale SP anomalies were identified. The summit area, particularly the 1962-63 vent area, showed strong positive anomaly, suggesting the upwelling of the magmatic fluid. While SP profiles at the southern slope showed simple “V”-shaped profile, those at the northern slope showed relatively high potentials. It may be influenced not only by different subsurface hydrothermal flows but by different geological structures.