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Repeated drone aeromagnetic surveys to reveal shallow thermal activity at Tokachidake Volcano

Toshiaki Hokari 1, Ryo Tanaka2, Takeshi Hashimoto2, Takao Koyama3

  • Affiliations: 1Graduate School of Science, Hokkaido University, Sapporo, Japan; 2Institute of Seismology and Volcanology, Hokkaido University, Sapporo, Japan; 3Earthquake Research Institute, The University of Tokyo, Tokyo, Japan

  • Presentation type: Poster

  • Presentation time: Thursday 16:30 - 18:30, Room Poster Hall

  • Poster Board Number: 35

  • Programme No: 3.8.19

  • Theme 3 > Session 8

Abstract

Magnetic surveys provide insights into subsurface temperature changes and rock alteration. Repeated aeromagnetic surveys are useful for detecting subsurface thermal activity in volcanoes (Koyama et al., 2022). Recently, drone-borne magnetic surveys have become practical, enabling researchers to collect aeromagnetic data independently and with flexibility. In this study, we conducted repeated drone aeromagnetic surveys at Tokachidake Volcano in Hokkaido, Japan, to identify ongoing thermal activity hidden in the shallow subsurface. Currently, geothermal anomalies are observed in regions slightly away from the crater of Tokachidake. We collected aeromagnetic data around the crater at a constant clearance above the ground using a drone in 2023 and 2024. From these datasets, we retrieved temporal changes in magnetic anomalies and investigated three-dimensional magnetization changes in the shallow subsurface. We modeled the distribution of magnetization changes with a 3D inversion code by Koyama et al. (2021). The model imaged a demagnetized region in the shallow part of the geothermal anomalies, likely caused by heating or alteration of subsurface rocks due to hydrothermal fluid intrusion. Conversely, a remagnetized area was imaged beneath the crater, possibly reflecting the cooling of subsurface rocks due to a decrease in the supply of hydrothermal fluids to the crater. These results suggest a shift in hydrothermal fluid flow, with a new supply route forming toward the geothermal anomalies. This shift may weaken subsurface structures through hydrothermal alteration, increasing the risk of sector collapse.