[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

Nagi Yamashita¹, *Tada-nori Goto¹, Keiichi Ishizu², Rei Amano¹, Souma Yamamoto¹, Yasuhiro Sugitani¹, Nana Fujiwara¹, Naoki Takahashi¹, Mao Monden¹, Satoru Yamaguchi³, Koki Aizawa⁴ (1.Graduate School of Science, University of Hyogo, 2.Graduate School of Engineering, Kyushu University, 3.Graduate School of Science, Osaka City University, 4.Graduate School of Science, Kyushu University)

Keywords:magnetotellurics, active fault, Futagawa fault, resistivity

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.