Magma reservoir beneath Azumayama Volcano, NE Japan, as inferred from a three-dimensional electrical resistivity model explored by means of magnetotelluric method

Abstract

An electrical resistivity model beneath Azumayama Volcano, NE Japan, is explored using magnetotelluric method to probe the magma/hydrothermal fluid distribution. Azumayama is one of the most concerning active volcanoes capable of producing a potential eruption triggered by the 2011 Tohoku-Oki Earthquake. The three-dimensional resistivity model reveals a conductive magma reservoir (< 3 Ωm) at depths of 3–15 km below sea level (bsl). The 67% and 90% confidence intervals of resistivity are 0.2–5 Ωm and 0.02–70 Ωm, respectively, for the magma reservoir. We assumed dacitic melt + rock at a shallow depth of 4 km bsl and andesitic melt + rock at a greater depth of 9 km bsl. The confidence interval of resistivity cannot be explained by using dacitic melt + rock condition at a depth of 4 km bsl. This suggests that very conductive hydrothermal fluids coexist with dacitic melt and rock in the shallow part of the magma reservoir. For the depth of 9 km bsl, the 67% confidence interval of resistivity is interpreted as water-saturated (8.0 weight %) andesitic melt–mafic rock complex with melt volume fractions greater than 4 volume %, while the shear wave velocity requires the fluid and/or melt volume fraction of 6–7 volume % at that depth. Considering the fluid and/or melt volume fraction of 6–7 volume %, the conductive hydrous phase is likewise required to explain the wide range of the 67% confidence interval of resistivity. The Mogi inflation source determined from geodetic data lies on the resistive side near the top boundary of the conductive magma reservoir at a depth of 2.7 or 3.7 km bsl. Assuming that the resistivity of the inflation source region is above the upper bound of the confidence interval of resistivity for the conductive magma reservoir and that the source region is composed of hydrothermal fluid + rock, the resistivity of the source region is explained by a hydrothermal fluid volume fraction below 5 volume %, which is the percolation threshold porosity in an effusive eruption. This indicates that the percolation threshold characterizes the inflation source region.

Trans-crustal structural control of CO2-rich extensional magmatic systems revealed at Mount Erebus Antarctica

“…This illustrates a fundamental difference between CO 2 -dominated volcanoes and the more familiar H 2 O-dominated subduction arc systems. Low-resistivity zones under arc volcanoes are typically confined to depths greater than 5 km, as exemplified by Mounts Ruapehu 55 , St Helens 35 , 39 , 56 , Tongariro 43 , 57 , Naruko 42 , and Azumayama 58 . Coincident seismic low velocity or high attenuation structures in the middle crust have been widely reported 58 – 62 .…”

“…Low-resistivity zones under arc volcanoes are typically confined to depths greater than 5 km, as exemplified by Mounts Ruapehu 55 , St Helens 35 , 39 , 56 , Tongariro 43 , 57 , Naruko 42 , and Azumayama 58 . Coincident seismic low velocity or high attenuation structures in the middle crust have been widely reported 58 – 62 . The depth of magma bodies beneath subduction zone volcanoes may be controlled by the amount of H 2 O that is dissolved in the magma 63 , 64 , although rheology could play a role 65 .…”

Estimation of Spatial Distribution and Fluid Fraction of a Potential Supercritical Geothermal Reservoir by Magnetotelluric Data: A Case Study From Yuzawa Geothermal Field, NE Japan

“…Therefore, C 1 at a depth >4 km bsl can contain melt. Resistivity models are useful for inferring the melt fraction in the subsurface (Feucht et al., 2017; Hata et al., 2015; Hill et al., 2009; Ichiki et al., 2021; Lee et al., 2020; Peacock et al., 2015; Piña‐Varas et al., 2018; Samrock et al., 2018). The resistivity of the melt has been well studied in the literature (Gaillard & Marziano, 2005; Guo et al., 2017; Laumonier et al., 2015; Pommier & Le‐Trong, 2011).…”

Special issue “Understanding phreatic eruptions - recent observations of Kusatsu-Shirane volcano and equivalents -”

“…This special issue also includes studies on phreatic eruptions at other volcanoes in Japan, New Zealand, and Costa Rica, and the summaries are described below. Ichiki et al (2021) obtained wideband magnetotelluric data around the Azumayama volcano, northeastern Japan, and imaged the magmatic-hydrothermal system from the three-dimensional inversion. They detected a conductor (less than 3 Ωm) at 3-15 km below sea level.…”