Large-scale magmatic–hydrothermal system of Kusatsu-Shirane Volcano, Japan, revealed by broadband magnetotellurics

Abstract

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.