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[SCG48-03] A three-dimensional electrical resistivity model beneath Azumayama Volcano, NE Japan
Keywords:magma reservoir, magnetotellurics, electrical resistivity, melt fraction, porosity, percolation threshold
We propose a three-dimensional (3-D) electrical resistivity model beneath Azumayama Volcano, NE Japan. The 3-D model shows the conductor beneath Oana Crater indicating magma reservoir. The dimension of the conductor less than 3 Ωm is about 15 km×10 km×10 km. The top depth is about 3 km below sea level. The 67 % and 90 % confidence intervals of resistivity (CIR) for the region less than 3 Ωm are 0.2–5 Ωm and 0.02–70 Ωm, respectively. Two checkerboard resolution tests using 1 Ωm cubes of the side of 3 and 5 km in 1000 Ωm background medium show that the resistivity of 5 km cubes are correctly estimated, while that the resistivity of 3 km cubes are estimated to be 3 to 10 Ωm.
Assuming melt + rock complex, we test whether 67 % CIR is able to be explained quantitatively. In addition, we postulate dacitic melt in shallower part (e.g. 3 to 5 km) and mafic melt in deeper part (e.g. deeper than 5 km). The result is CIR cannot be explained in the shallower part condition even assuming a water-saturated (5.5 wt %) dacitic melt. Thus, we interpret the shallower part conductor consists of dacitic melt-silicic rock-hydrothermal fluid complex. On the other hand, CIR can be estimated by water-saturated (8 wt %) andesitic melt-mafic rock complex.
The Mogi inflation source determined from GNSS and tilt data is located near the top boundary of the conductor at a depth of 2.7–3.7 km (Seki et al., 2021, Q. J. Seismology, under review), which suggests that the ascent of hydrothermal fluids exsolved from the dacitic melt is interrupted by the impermeable roof. Assuming two phases of hydrothermal fluid and silicic rock, the resistivity at the inflation source corresponds to about 5 % of hydrothermal fluid fraction. The 5 % is percolation threshold porosity of effusive eruption (e.g. Colombier et al., 2017 EPSL). This indicates that the percolation threshold porosity characterizes the impermeable roof associated with the Mogi inflation source.
Assuming melt + rock complex, we test whether 67 % CIR is able to be explained quantitatively. In addition, we postulate dacitic melt in shallower part (e.g. 3 to 5 km) and mafic melt in deeper part (e.g. deeper than 5 km). The result is CIR cannot be explained in the shallower part condition even assuming a water-saturated (5.5 wt %) dacitic melt. Thus, we interpret the shallower part conductor consists of dacitic melt-silicic rock-hydrothermal fluid complex. On the other hand, CIR can be estimated by water-saturated (8 wt %) andesitic melt-mafic rock complex.
The Mogi inflation source determined from GNSS and tilt data is located near the top boundary of the conductor at a depth of 2.7–3.7 km (Seki et al., 2021, Q. J. Seismology, under review), which suggests that the ascent of hydrothermal fluids exsolved from the dacitic melt is interrupted by the impermeable roof. Assuming two phases of hydrothermal fluid and silicic rock, the resistivity at the inflation source corresponds to about 5 % of hydrothermal fluid fraction. The 5 % is percolation threshold porosity of effusive eruption (e.g. Colombier et al., 2017 EPSL). This indicates that the percolation threshold porosity characterizes the impermeable roof associated with the Mogi inflation source.