Abstract
We investigated the magmatic–hydrothermal system of Aso Volcano, Japan, using broadband magnetotelluric (MT) data. To establish the nature of the shallow crust, a previous resistivity model based on data from 100 measurement sites in and around Aso volcano was revised using data from 9 additional sites near Naka-dake crater, which is located in the central part of the volcano. The components of MT impedance and the tipper vector were used to obtain the resistivity structure by three-dimensional inversion. The resistivity structure shows a subvertical low-resistivity (< 1 Ωm) column-shaped body beneath Naka-dake crater that extends from − 600 m to 10 km below sea level (BSL) and dips steeply to the north-northeast. The position of the upper part of the column is displaced eastward compared with the previous model and does not overlap the position of the presumed magma reservoir inferred previously from seismic and geodetic observations underneath the western side of Naka-dake crater at a depth of 5 km. We interpret this low-resistivity column to be a magmatic–hydrothermal system composed of brine and magma that were transported to Naka-dake crater from the main deep-seated magma reservoir. A horizontal low-resistivity (< 10 Ωm) layer occurs beneath post-caldera cones at the depths of 0–2 km BSL, and this layer extends laterally from the upper part of the low-resistivity column. We interpret this low-resistivity layer as representing a shallow hydrothermal system that has developed around the central column-shaped magmatic–hydrothermal system.
Highlights
Determining the source and location of a magmatic– hydrothermal system is important for understanding volcanic activity
The sill-like inflation source of crustal deformation detected using global navigation satellite system (GNSS) data (Geospatial Information Authority of Japan (GSI 2004)) and the deep, low-frequency earthquakes observed by the Japan Meteorological Agency (JMA) are located just below this low-resistivity column
Resistivity of magma The low-resistivity column identified in the present study is subvertical, dips steeply to the north-northeast, and trends toward the sill-like source of crustal deformation of GSI (2014) and the distribution of low-frequency earthquakes that occur below 15 km below sea level (BSL) (JMA earthquake catalog)
Summary
Determining the source and location of a magmatic– hydrothermal system is important for understanding volcanic activity. Geophysical methods such as seismic data analysis, crustal deformation monitoring, and magnetotelluric (MT) surveys are effective tools for investigating the existence of magma within a volcano and associated hydrothermal systems (e.g., Nakamichi et al 2007; Heise et al 2010; Siena et al 2010; Seccia et al 2011; Lin et al.2013; Luttrell et al 2013; Farrell et al 2014; Aizawa et al 2014; Ogawa et al 2014). On the basis of these observations, Hata et al (2016, 2018) interpreted the low-resistivity column as a magma supply system connecting the magma reservoirs
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