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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. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Kusatsu-Shirane Volcano (KSV) is an active volcano in the central Honshu Arc, Japan, and is known for its significant hydrothermal activity and repeated phreatic eruptions over the past 200 years. On the basis of its long-term volcanic activity, an extensive magmatic–hydrothermal system is considered to have developed within the volcano, with strong impacts on volcanic processes. Previous magnetotelluric studies conducted at KSV have successfully revealed the structure of the hydrothermal system, mainly beneath the summit area. However, as the deep structure was still unclear, there was no information on the location of magma storage as a heat source to drive volcanic activity. Here we report a revised three-dimensional electrical resistivity structure model for KSV down to a depth of ~10 km below sea level (bsl), inferred from broadband magnetotelluric data (256–0.004 Hz) collected over a wider area than previous studies. The most notable feature of the model is a sub-vertical conductive zone that extends from a depth of 1.5 km below the summit area to a depth of ~10 km in the north-northwestern part of KSV. We interpret resistivity values to infer the upper half of the conductive zone (<4 km bsl) as a zone containing high-salinity brine and the lower half (>4 km bsl) as a partially molten zone. The fact that the inflation source during the unrest periods is located just beneath the brine zone indicates that the lower partially molten zone contains degassing magmas. This partially molten zone extends beneath the adjacent Shiga Volcano, suggesting that the two volcanoes could share a common magma source. The brine zone in the upper half is considered to have formed as a result of repeated magmatic intrusions during the Stage III activity of KSV since 17 ka. As this brine zone is not found beneath the presently inactive Shiga Volcano, it is considered that the formation of brine zones could be used as an indicator for evaluating recent magmatic activity and potential volcanic hazards. • Resistivity structure of Kusatsu-Shirane Volcano down to ~10 km depth was revealed. • Highly conductive zone thought to contain brine was found at a depth of 1.5-6 km. • Conductor found beneath the brine zone suggests the presence of a partial melt. • Brine zone is likely to have been formed by the magmatic activity after 17 ka. [ABSTRACT FROM AUTHOR]

Abstract: The 1st crater of Naka-dake, Aso volcano, is one of the most active craters in Japan, and known to have a characteristic cycle of activity that consists of the formation of a crater lake, drying-up of the lake water, and finally a Strombolian-type eruption. Recent observations indicate an increase in eruptive activity including a decrease in the level of the lake water, mud eruptions, and red hot glows on the crater wall. Temporal variations in the geomagnetic field observed around the craters of Naka-dake also indicate that thermal demagnetization of the subsurface rocks has been occurring in shallow subsurface areas around the 1st crater. Volcanic explosions act to release the energy transferred from magma or volcanic fluids. Measurement of the subsurface electrical resistivity is a promising method in investigating the shallow structure of the volcanic edifices, where energy from various sources accumulates, and in investigating the behaviors of magma and volcanic fluids. We carried out audio-frequency magnetotelluric surveys around the craters of Naka-dake in 2004 and 2005 to determine the detailed electrical structure down to a depth of around 1 km. The main objective of this study is to identify the specific subsurface structure that acts to store energy as a preparation zone for volcanic eruption. Two-dimensional inversions were applied to four profiles across the craters, revealing a strongly conductive zone at several hundred meters depth beneath the 1st crater and surrounding area. In contrast, we found no such remarkable conductor at shallow depths beneath the 4th crater, which has been inactive for 70 years, finding instead a relatively resistive body. The distribution of the rotational invariant of the magnetotelluric impedance tensor is consistent with the inversion results. This unusual shallow structure probably reflects the existence of a supply path of high-temperature volcanic gases to the crater bottom. We propose that the upper part of the conductor identified beneath the 1st crater is mainly composed of hydrothermally altered zone that acts both as a cap to upwelling fluids supplied from deep-level magma and as a floor to infiltrating fluid from the crater lake. The relatively resistive body found beneath the 4th crater represents consolidated magma. These results suggest that the shallow conductor beneath the active crater is closely related to a component of the mechanism that controls volcanic activity within Naka-dake. [Copyright &y& Elsevier]