[S-VC35] Hydrothermal systems of volcanoes

[SVC35-06] A confidence interval inference method of an electrical resistivity model and a smectite content estimation from the resistivity confidence interval beneath Zao volcano

*Masahiro Ichiki¹, Wataru Kanda², Toshiki Kaida¹, Masashi Ushioda^2,3, Kaori Seki^2,4, Satoshi Miura¹, Mare Yamamoto¹, Yuichi Morita^5,6, Makoto Uyeshima⁵ (1.Graduate School of Science, Tohoku University, 2.Institute of Integrated Research, Institute of Science Tokyo, 3.Department of Civil Engineering, Shikoku Research Institute Inc., 4.Meteorological Research Institute, Japan Meteorological Agency, 5.Earthquake Research Institute, The University of Tokyo, 6.Center for Integrated Volcano Research, National Research Institute for Earth Science and Disaster Resilience)

Keywords:electrical resistivity, confidence interval of resistivity, smectite, phreatic eruption

The electrical resistivity structures beneath active volcanoes commonly reveal a shallow electrically conductive layer at a depth above sea level and a conductive body at 0 to 3 km depth below sea level (BSL) (e.g., Tsukamoto et al., 2018; Yoshimura et al., 2018; Tseng et al., 2020). The shallow conductive layer is considered a hydrothermal alteration and works as the cap rock to prevent hydrothermal fluids from upwelling. The deeper conductive body is interpreted as a magma and/or hydrothermal reservoir. To investigate the hydrothermal alteration layer’s role in phreatic eruption, we evaluate how much conductive shallow layer indicates a content of impermeable clay mineral or smectite and discuss a mechanism such that temperature raising turns the impermeable layer to be permeable. We chose Zao Volcano, NE Japan, as a test field and modeled the subsurface resistivity structure down to a depth of 2 km BSL. We inverted the magnetotelluric impedances and geomagnetic transfer functions in 1-10k Hz at 60 stations in a 1 km × 1 km area from east to west, north to south centered on the crater lake, Okama, Zao Volcano. The resultant three-dimensional resistivity model represents a conductive layer within ±1 km from east to west, north to south, centered on Okama, down to about 1000 m depth below the surface. The shallow conductive zone has a low resistivity in 1-10 Ohm-m. To restrict the smectite content of the conductive layer, we estimated the confidence interval of the resistivity (CIR) with a 99 % level to be 1.5-2.5 Ohm-m using student’s t-test. The student’s t-test is based on the central limit theorem and assesses the significance of the average change of the raw misfit of magnetotelluric impedance and geomagnetic transfer function. Assuming 150 ℃ at temperature, the confidence interval of 1.5-2.5 Ohm-m corresponds to less than 9.5 volume % in smectite (Levy et al., 2018; Revil et al., 2019). In the presentation, we will show a time scale of the smectite transformation into illite or chrolite, depending on the temperature rising.