Abstract

During the past two decades, studies of the Aso volcano in Japan have improved our understanding of the shallow hydrothermal system that exists beneath the active crater of this volcano. Detailed knowledge of the subsurface structure of this volcanic edifice is essential for developing a better understanding of the behavior of the volcanic fluids and of the triggering mechanism of volcanic eruptions. Here, we report a three-dimensional (3-D) electrical resistivity model for the active crater of the Nakadake central cone of Aso volcano using audio-frequency magnetotelluric (AMT) data obtained during 2004–2005. The AMT data were collected at 43 sites on a grid (distance between grid points: ~ 300 m) around the crater. However, as yet, only two-dimensional sectional resistivity models have been generated for this survey area. Using 3-D inversion, we obtain a resistivity model that shows similar characteristics to those of the 2-D models. A highly conductive zone is observed beneath the active crater down to a depth of approximately 300 m. Based on the recent findings regarding the shallow hydrothermal system of the volcano, we interpret this conductive zone to have been formed by highly conductive acidic fluids filling a fractured region. This view modifies the past interpretation made on the 2-D models and promotes understanding of fluid behavior beneath the active crater.

Highlights

  • Knowledge on the subsurface structure is essential for a better understanding of the volcanic activity

  • The obtained resistivity model shows similar characteristics to those of the 2-D cross sections presented by Kanda et al (2008)

  • Combining our results with recent findings regarding the hydrothermal system of the volcano, we infer that the region from the upper end of the cracklike conduit to the bottom of the first crater exhibits high conductivities because this region is highly fractured and filled with acidic hydrothermal fluids

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Summary

Introduction

Knowledge on the subsurface structure is essential for a better understanding of the volcanic activity. The triggering mechanism and/or the style of eruptions greatly depend on whether the fluid within the volcanic edifice is involved or not (e.g., Schmincke 2004). Distribution and behavior of the fluid within a volcano is regulated by the hydrothermal system developed in the shallow subsurface, and electrical resistivity has often been used to determine its characteristics (e.g., Pellerin et al 1996). Aso volcano in southwestern Japan (Fig. 1a, b) has registered repeated eruptions from the first crater of Nakadake, one of the volcano’s central cones, over the past 80 years. As the volcanic activity increases, the crater lake gradually dries as a result of heat supplied from the deep subsurface, and a red-hot glow becomes visible in the bottom or wall of the crater.

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