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.
Highlights
Great earthquakes have often triggered eruptions in neighboring volcanoes (Bebbington and Marzocchi 2011; Koyama 2015; Nishimura 2017)
The primary purpose of this study is to model the three-dimensional resistivity distribution down to a depth of 20 km bsl, in order to provide a clear image of the magma reservoir beneath Azumayama
The Checkerboard resolution test (CRT) suggest the dataset can resolve a conductive body over 5 km in scale with unbiased resistivity at depths of 5 to 10 km bsl (6.7 to 11.7 km bsf )
Summary
Great earthquakes have often triggered eruptions in neighboring volcanoes (Bebbington and Marzocchi 2011; Koyama 2015; Nishimura 2017). While the potential eruptions of these volcanoes have raised concerns, Azumayama Volcano is one of the most concerning of the active volcanoes in NE Japan triggered by the 2011 Tohoku-Oki Earthquake. Because this volcano is close (~ 250 km) to the hypocenter of the 2011 Tohoku-Oki Earthquake, and sporadic seismic swarms have been observed around this volcano since the year 2000. Outstanding seismic swarms were observed during the years 2014– 2015 and 2018–2019 (Japan Meteorological Agency 2019; Yoshigai et al 2019), no eruption has taken place so far. The recent phreatic eruptions occurred in Oana Crater in the year 1977, 1952, and 1950 (Kawanabe and Ueki 2013)
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