3-D resistivity structure model around Kusatsu-Shirane Volcano revealed by broadband magnetotellurics

Abstract:

Kusatsu-Shirane Volcano (KSV) is the active volcano known for its significant hydrothermal activity and for producing phreatic eruptions in recent years. Several geochemical and geophysical studies, conducted around the present active eruption center (Yugama), have revealed the shallow hydrothermal system mainly around the crater. According to geological studies, there were also frequent magmatic eruptions at Mt. Motoshirane, a different pyroclastic cone from that which hosts Yugama, until 1500 years ago. Therefore, it is expected that the magma erupted at that time was not yet cooled and solidified. However, the location of a magma chamber is still unknown because of a lack of deep structural information. Knowing the subsurface structure such as the spatial distribution of the magma-hydrothermal system is essential to discuss the present volcanic activity and the future eruption risk.

As a first step to reveal the whole image of the magma-hydrothermal system of KSV, broadband magnetotelluric (MT) observations centered on Mt. Motoshirane were conducted in 2015 and 2016 (Matsunaga et al., 2020). The 3-D electrical resistivity structure model revealed an extensive low-resistivity layer which was interpreted as a hydrothermal reservoir supplying fluids to the active crater at depths of 1–3 km beneath the summit area. However, no structural features suggestive of the presence of magma have been found beneath this layer. This is because the resolution of the structure deeper than 5 km was not sufficient.

Since 2019, we have conducted additional MT observations covering the entire edifice of KSV. By expanding the observation area to a wider range, it is expected that a more reliable deep resistivity structure will be obtained. The preliminary structure model inferred using the MT data from 34 sites recorded in 2019 shows that the low-resistivity layer beneath the summit area, which is also revealed by 2015-2016 data, is extending towards the deep part in the north of KSV. Since hypocenters widely distributed on the top of this low-resistivity layer, the upper part of the layer could be interpreted as the hydrothermal reservoir which is the same as the interpretation of results from the 2015-2016 data. The deeper part of the layer may reflect the presence of the magma chamber, however, its exact position is still unknown. In the presentation, we are going to present anrevised 3-D resistivity structure of KSV inferred using all MT data since 2015, and to discuss the spatial distribution of the magma-hydrothermal system.