The study of applicability of the geophysical exploration methods for the strike-slip fault (part 1; shallow P-wave seismic reflection and refraction survey, CSAMT survey, and gravity survey) : the case study of the Go-mura fault zone and Yamada fault zone

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  • 横ずれ断層における各種物理探査の適用可能性の検討（その1：浅層反射法地震探査・屈折法地震探査・CSAMT探査・重力探査）―郷村断層帯および山田断層帯における事例―
  • ヨコズレ ダンソウ ニ オケル カクシュ ブツリ タンサ ノ テキヨウ カノウセイ ノ ケントウ(ソノ 1)センソウ ハンシャホウ ジシン タンサ ・ クッセツホウ ジシン タンサ ・ CSAMT タンサ ・ ジュウリョク タンサ : ゴウソン ダンソウタイ オヨビ ヤマダ ダンソウタイ ニ オケル ジレイ

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Abstract

<p>The geometry/angle of the deeper extension of active fault is one of the fundamental information to evaluate the magnitude of inland earthquake associated with active faulting. The information is mainly revealed by geophysical exploration that is executed across the active fault. Although the geophysical explorations were scarcely applied for strike-slip fault in Japan, we discuss the applicability of several geophysical explorations, seismic reflection and refraction survey, CSAMT (controlled source audio-magnetotelluric) survey and gravity survey for strike-slip fault in this study. The study area is located in northwest area in Kinki region in Japan, and Go-mura fault zone and Yamada fault zone are distributed in a granite area (Miyazu granite). In the seismic reflection profile, discontinuity and inflection of reflector shows the deeper geometry of active fault in 200-300 m below the surface of the ground. In some profile, discordance between subsurface structure and slip direction of the active fault were shown. Hence, the comparison between seismic reflection profile and other geophysical explorations is important in the interpretation of subsurface geometry of the active fault. Although, the seismic refraction image cannot define the strict angle of the active fault in principle, the image well indicates the low-velocity layer crushed by the strike-slip faulting. In the CSAMT survey, low-specific resistance area extended in depth clearly shows the geometry and angle of deeper part of active fault. In some image, pseudo-specific resistance can be seen in some CSAMT image, comparison to other information of geophysical survey is necessary for interpretation. Because the vertical displacement is not dominant in strike-slip faulting, the gravity survey across the strike-slip fault is generally insufficient method to detect the fault. Dense spacing of gravity stations and precise measurement allow sensing the Bouguer gravity anomaly associated with strike-slip faulting.</p>

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