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

[SEM15-04] Exposed and buried faults in the near-surface resistive structure inferred by a high-density audio-frequency magnetotelluric survey around the Futagawa fault, Japan

山下 凪¹、*後藤 忠徳¹、石須 慶一²、天野 玲¹、山本 壮馬¹、杉谷 安優¹、藤原 奈々¹、髙橋 直暉¹、門田 真桜¹、山口 覚³、相澤 広記⁴ (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.大阪市立大学大学院理学研究科、4.九州大学大学院理学研究科)

キーワード：MT探査、活断層、布田川断層、比抵抗

Fault core and damage zones along active faults have low resistivity due to clay minerals and water in small-scale fractures. The electrically conductive faults can be visualized by magnetotelluric (MT) method. However, the detailed three-dimensional (3D) fault structures have been rarely reported due to the limitation of spatial resolution of MT surveys. In this study, we conducted spatially high-density audio-frequency MT (AMT) survey around the Futagawa fault, where distinct surface ruptures were observed at the in 2016 Kumamoto earthquake. AMT data was obtained at about 80 sites. By using an inversion algorithm, 3D resistivity structure from the surface to the depth about 1 km was estimated. At about 100 m below the surface, we found a scattered distribution of small low resistive bodies along the surface fracture. At the deeper depth (400m below the surface), two conductive bodies are found; one is located near the surface trace of the Futagawa fault. Another one corresponds the linear surface displacement detected by InSAR images, while it does not accompany any known faults. These low resistive bodies cannot be explained by the dry rocks, so that they indicate the high water/clay content in fractures along the known and unkwon (burried) active faults. Thus, the high-density AMT survey is effective to reveal the known and buried fault systems.