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.

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从电阻率结构推断日本阿苏火山的岩浆热液系统

我们使用宽带大地电磁 (MT) 数据研究了日本阿苏火山的岩浆热液系统。为了确定浅地壳的性质，使用位于阿苏火山中央部分的中岳火山口附近的另外 9 个站点的数据修改了基于阿苏火山内部和周围 100 个测量站点数据的先前电阻率模型。利用MT阻抗分量和倾角矢量，通过三维反演得到电阻率结构。电阻率结构显示在 Naka-dake 火山口下方有一个近垂直的低电阻率 (< 1 Ωm) 柱状体，从海平面以下 (BSL) - 600 m 延伸至 10 km，并急剧下降至东北偏北。与之前的模型相比，柱子上部的位置向东移动，并且与之前根据中岳火山口西侧下方 5 公里深度的地震和大地测量推断的推定岩浆储层的位置不重叠. 我们将这个低电阻率柱解释为一个由盐水和岩浆组成的岩浆-热液系统，它们从主要的深部岩浆储层输送到 Naka-dake 火山口。水平低电阻率 (< 10 Ωm) 层出现在火山口后锥体下方 0-2 公里 BSL 深度处，该层从低电阻率柱的上部横向延伸。我们将这个低电阻率层解释为代表一个围绕中央柱状岩浆热液系统发育的浅层热液系统。