Spatial extent and eruptive potential of hydrothermal system at Numanotaira Crater, Adatara Volcano, Japan, inferred from shallow resistivity structure and diffuse soil gas flux measurement

Abstract The Numanotaira Crater (NC) of the Adatara Volcano is located in northern Fukushima Prefecture, northeastern Japan, and has repeatedly undergone phreatic eruptions and volcanic lahars. Recent eruptions occurred between 1899 and 1900, and hydrothermal activity has continued since then. Based on previous geophysical and geological studies, a well-developed hydrothermal system is believed to have formed beneath the crater. However, the subsurface structure of shallow hydrothermal systems has not been investigated. In this study, an audio-frequency magnetotelluric survey was conducted in and around NC to clarify the distribution of shallow hydrothermal systems and understand the relationship between the subsurface structure and recent hydrothermal activities to examine the potential for phreatic eruptions at NC. In addition, several surveys (diffuse CO 2 flux, ground temperature, and resistivity of hot spring and river water) were conducted over a wide area of NC to assist in interpreting the resistivity structure. As a result of three-dimensional resistivity inversion analysis considering the steep topography around NC, the following findings were obtained. A low-resistivity zone was found beneath the crater floor, which had previously been the center of hydrothermal activity. However, part of this zone was overlain by a high-resistivity and low-permeability zone near the surface. The presence of a sulfur layer or native sulfur veins is likely to have caused this low-permeability zone. Low-resistivity zones were also found around the crater floor. In particular, the location of the low-resistivity zone beneath the southern slope of the inner crater wall was consistent with that of a pressure source. This may indicate the formation of a cap structure that seals the upwelling hydrothermal fluids. At greater depths, the low-resistivity zone extended westward, consistent with the distribution of hydrothermally altered zones and hot springs. Considering the results of water resistivity measurements along the Iwo River, which confirmed hydrothermal fluid seepage at several locations, the hydrothermal system beneath NC could be continuous to the west. High-resistivity bodies, interpreted as unaltered rocks, were found in the northern and southern parts of NC, suggesting that these rocks constrained the past development of the hydrothermal system of NC. Graphical Abstract