[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

土方 歳志¹、*後藤 忠徳¹、山下 凪¹、櫻井 未久¹、天野 玲¹、小田 彩湖¹、山本 壮馬¹、山本 睦徳³、石須 慶一² (1.兵庫県立大学大学院理学研究科、2.九州大学大学院工学研究院、3.大阪市立自然史博物館)

キーワード：AMT探査、断層、カルデラ

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.