[S-VC28] Hydrothermal systems of volcanoes

[SVC28-P07] Estimation of pressure in the hydrothermal reservoir beneath the Jigokudani Valley, Midagahara volcano

*Tohru Watanabe¹, Keito Yonekura² (1.Faculty of Sustainable Design, University of Toyama, 2.School of Sustainable Design, University of Toyama)

Keywords:phreatic eruption, Midagahara volcano, pressure, fumes

The Jigokudani Valley in Midagahara volcano is an active geothermal field with numerous fumaroles and hot springs. It was formed by repeated phreatic eruptions over 40,000 years. A magnetotelluric survey has revealed a hydrothermal reservoir ~100 m below the Jigokudani Valley (Seki et al., 2015; 2016). This reservoir combines hydrothermal fluids expelled from the deep-seated magma with the ground water to supply volcanic gases and hydrothermal fluids to fumaroles and hot springs (Seki et al., 2016; 2019). In a phreatic eruption, high-pressure vapor blows subsurface materials to create an explosion crater. The vapor pressure must be higher than 0.3 MPa to create a crater with the diameter of 10 m. The estimation of the pressure in the hydrothermal reservoir is thus critical to assess the potential of a phreatic eruption. The pressure in the reservoir is estimated from the outflow speed of fumes of the order of 1 m/s. Fumaroles are assumed to be connected to the reservoir through a vertical circular pipe. Because of high Reynolds number, equations for turbulent flow were used. The pressure in the reservoir is estimated to be higher than the atmospheric pressure by only 300 Pa and must be increased by sealing the conduits before a phreatic eruption. A crack filled with hydrothermal fluid which is directly supplied from a deep-seated magma may have high-pressure vapor to cause a phreatic eruption.