[S-EM15] Electric, magnetic and electromagnetic survey technologies and scientific achievements

[SEM15-P11] Subsurface resistivity structure in a caldera in the Iburi Province, Hokkaido, Japan

Toshi Hijikata¹, *Tada-nori Goto¹, Nagi Yamashita¹, Miku Sakurai¹, Rei Amano¹, Ayako Oda¹, Souma Yamamoto¹, Mutsunori Yamamoto³, Keiichi Ishizu² (1.Graduate School of Science, University of Hyogo, 2.Graduate School of Engineering, Kyushu University, 3.Osaka City Museum of Natural History)

Keywords:AMT survey, Fault, Caldera

In calderas, hydrothermal systems are well developed due to the presence of numerous faults and fractures. Previous studies have reported the various influences of the caldera faults on the hydrothermal circulation. However, a unified model of the circulation in calderas has yet to be established. In this study, we conducted a high-density audio-frequency magnetotelluric (AMT) survey in a small caldera located in the Iburi region of Hokkaido. The AMT survey can visualize subsurface hydrothermally altered zones, as they exhibit extremely low resistivity, allowing fractures with upwelling hydrothermal fluid to be clearly imaged. Three-dimensional (3D) inversion analysis of the AMT data revealed an elongated low-resistivity zone in the center of the caldera. The strike direction of this low-resistivity zone approximately corresponds to that of the regional topographic lineaments, suggesting that it represents an upward flow zone along the fracture system. On the other hand, no distinct low-resistivity bodies were observed along the caldera walls, where numerous normal faults are likely to exist. These findings indicate that the subsurface hydrothermal system is primarily controlled by fault systems developed at the central part of the caldera, and the normal faults along the caldera wall may correspond to the recharge areas.