Shallow thermal activity of Tokachidake Volcano (Japan) investigated through repeated drone-borne magnetic surveys

Abstract In recent years, new geothermal anomalies have been detected on the northwestern slope of the 62-II crater near the summit of Tokachidake Volcano, Hokkaido, Japan. These surface manifestations imply the formation of new shallow subsurface pathways for hydrothermal fluids, accompanied by localized heating and progressive hydrothermal alteration along these pathways. Because hydrothermal alteration can weaken volcanic rocks and contribute to sector collapse, identifying the distribution of alteration and the associated temperature changes within the edifice is crucial for assessing potential collapse hazards. This study aimed to visualize the shallow thermal activity beneath Tokachidake Volcano using drone-borne magnetic surveys. Magnetic surveying provides insight into subsurface temperature changes and the extent of hydrothermal alteration. In particular, aeromagnetic surveys enable the efficient investigation of shallow volcanic thermal activity without terrain-related access limitations. We conducted drone-borne magnetic surveys in 2023, 2024, and 2025 to acquire geomagnetic field data around the geothermal anomalies at a constant clearance above the ground surface. From these datasets, we derived static magnetic anomalies that reflect the effects of topographic relief and subsurface heterogeneity, as well as their temporal differences. Three-dimensional inversion was then applied to estimate the distribution of subsurface magnetization intensity and its temporal variations. Static magnetization models revealed a columnar low-magnetization zone directly beneath the 62-II crater, representing the heated and hydrothermally altered regions surrounding the fumarolic conduit. Temporal magnetization variations revealed a remagnetized zone beneath the 62-II crater and a demagnetized zone beneath the geothermal anomalies, indicating the reorganization of shallow heat transfer associated with the development of new hydrothermal fluid pathways. The demagnetized zone beneath the Maetokachi slope suggests the presence of hydrothermal fluid pathways within the slope interior, where progressive alteration likely weakens the volcanic rocks and reduces slope stability. These findings provide new insights into the evolution of the shallow hydrothermal system of Tokachidake Volcano. The active redistribution of heat and fluids within the edifice appears to contribute to the internal weakening of the Maetokachi slope, potentially increasing the likelihood of future sector collapse. Continued aeromagnetic monitoring combined with hydrothermal and slope-stability modeling will be essential for effective hazard assessment. Graphical Abstract