Hydrothermal system of the active crater of Aso volcano (Japan) inferred from a three-dimensional resistivity structure model

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.

Anatomy of active volcanic edifice at the Kusatsu–Shirane volcano, Japan, by magnetotellurics: hydrothermal implications for volcanic unrests

“…In this paper, the distribution of the clay cap around the Yugama crater was imaged in detail, and it was found to be nearly bell shaped. These capping structures, the hydrothermal alteration products, are commonly found in geothermal areas (e.g., Heise et al 2008;Yamaya et al 2013;Seki et al 2015;Piña-Varas et al 2018;Yoshimura et al 2018;Kanda et al 2019). The bottom of the C1 smectite clay conductor is interpreted as the isotherm of 200 °C.…”

Electrical conductivity of HCl-bearing aqueous fluids to 700 ºC and 1 GPa

“…Unzen (Srigutomo et al 2008), Mt. Fuji (Aizawa et al 2005) and many others (e.g., Hoffmann-Rothe et al 2001;Yamaya et al 2009;Aizawa et al 2011;Díaz et al 2015;Kanda et al 2019). Conductive fluids may also allow the visualization of regional fault systems from magnetotelluric data (Pavez et al 2020).…”

“…From previous measurements (Frantz and Marshall 1984), it is known that the conductivity of aqueous HCl can be significantly larger than that of NaCl solutions. Indeed, Kanda et al (2019) attributed the elevated conductivity below Aso volcano in Japan to the presence of a highly acidic fluid.…”

Episodic transport of discrete magma batches beneath Aso volcano

“…Beneath Aso volcano, an basalt-andesitic volcano in Japan 21 – 23 , repetitive very long-period signals (VLP, ~15 s period) are known to occur in a fluid-filled crack-like conduit within a shallow hydrothermal aquifer 24 – 26 , ~1 km beneath the active Naka-dake first crater 27 – 32 (Fig. 1a ).…”