TY - JOUR

T1 - Magma reservoir beneath Azumayama Volcano, NE Japan, as inferred from a three-dimensional electrical resistivity model explored by means of magnetotelluric method

AU - Ichiki, Masahiro

AU - Kaida, Toshiki

AU - Nakayama, Takashi

AU - Miura, Satoshi

AU - Yamamoto, Mare

AU - Morita, Yuichi

AU - Uyeshima, Makoto

N1 - Funding Information:
This study is supported by the Ministry of Education, Culture, Sports, Science and Technology (MEXT) of Japan, under the Program of Mobile and Intensive Observation-Research System Integration for Volcanic Areas, and the Earthquake and Volcano Hazards Observation and Research Program (Earthquake and Volcano Hazard Reduction Research).
Funding Information:
Comments by the editor Yasuo Ogawa and two anonymous reviewers greatly improve this paper. We are grateful to Satoshi Okumura, Masao Ban, Yu Nihara, Yusuke Yoshigai, Jun Okada, Yo Fukushima, and the staff of Sendai Regional Headquarters of Japan Meteorological Agency for valuable comments and discussion. We thank Tomomi Okada and Alan Chave for providing their results of seismic wave velocity models and BIRRP processing code. Urabandai Environment Ranger Office, Fukushima and Yonezawa City Office, and Fukushima and Aizu Forest Office facilitated the observation. Geothermal Energy Research and Development Co. Ltd. and Nittetsu Mining Consultants Co. Ltd. kindly provided the remote reference data. Figures are created using Generic Mapping Tools and Paraview Software.
Publisher Copyright:
© 2021, The Author(s).

PY - 2021/12

Y1 - 2021/12

N2 - 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. [Figure not available: see fulltext.]

AB - 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. [Figure not available: see fulltext.]

KW - Electrical resistivity

KW - Hydrothermal fluid

KW - Magma reservoir

KW - Magnetotellurics

KW - Melt fraction

KW - Mogi inflation source

KW - Percolation threshold

KW - Permeability

KW - Shear wave velocity

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U2 - 10.1186/s40623-021-01451-y

DO - 10.1186/s40623-021-01451-y

M3 - Article

AN - SCOPUS:85111324534

SN - 1343-8832

VL - 73

JO - Earth, Planets and Space

JF - Earth, Planets and Space

IS - 1

M1 - 150

ER -