報告書番号

N08039

タイトル（和文）

横ずれ断層の変位に伴う岩盤の３次元変形過程-ヘリカルX線CTを用いた断層変位実験による検討-

タイトル（英文）

Deformation of rock mass caused by strike-slip faulting - 3D analysis of analogue models by helical X-ray computed tomography -

概要 （図表や脚注は「報告書全文」に掲載しております）

1971年にロサンゼルスの近郊で発生したサンフェルナンド地震において，断層変位による地盤の変形により多数の構造物が被害を受けて以来，地表および地下の重要構造物の立地・設計に際して，地震時の活断層の変位に伴う周辺地盤の変形の範囲とその程度を予測する手法の構築は，重要な課題となっている．本報告では模擬岩盤を用いた断層変位実験を行い，ヘリカルX線CTを用いた解析により，横ずれ断層の変位に伴う岩盤の3次元変形過程を解明した．また断層変位に伴う岩盤の変形を評価する上で，断層系の形態と活動性を各種調査で正確に把握し，断層系の成長過程（成熟度）と岩盤の物性・応力状態を考慮し評価を行うことが重要であることが明らかとなった．特に断層の屈曲部では直線部に比べフラクチャーゾーンの幅が広くなる傾向にあることに留意する必要がある．

概要 （英文）

Strike-slip fault zones are induced experimentally in artificial rock subjected to strike-slip displacement along basement fault. The purpose is to investigate in three dimensions, the geometries and sequence of development of structural elements comprising the fault zones by use of a helical X-ray CT scanner. 860 mm long, 310 mm wide, 18-30 mm high artificial rocks were made by mixing sand, plaster and water. The basement fault was displaced up to 100 mm at a displacement rate of 0.01-0.1mm/sec. The deformation of the artificial rocks with increasing basement displacement was observed as follows. 1) En echelon fractures corresponding to the Riedel shears are observed at the surface of the artificial rock. These Riedel structures contain within them similar Riedels on a smaller scale (Riedel within Riedel structures). In three dimensions, each fracture has helicoidal shape. 2) Fractures corresponding to the first and second order P-shears form at the junctions between two Riedel shears, and serve to connect the Riedel shears. The combination of displacement along the Riedel and P-shears leads to the formation of the principal displacement shears including first and second order jogs and pull-aparts. 3) New shears (Outer shears) branch off from Riedel and P-shears in compressional jogs and propagate aside from the fault zone that consists of Riedel and P-shears. The outer shears do not join the basement fault directly and develop near the surface of the artificial rock. The region among the Riedel shear, P-shear and outer shear is an up-squeezed block (push-up), which undergoes rotation with increasing displacement. The push-up structures tend to be limited to shallow part of the artificial rock. The lower artificial rock on the one side of basement fault adheres to one on the other side in the compressional jogs. 4) As slip proceeds, wear erode the jogs and produce the progressively more continuous and smoother principal displacement shear plane. Such evolution of the shears and its associated structures in the fault model tests agrees well with that of strike-slip fault systems and its associated geomorphic structures. The width of fracture zone strongly depends on the fault geometry, mechanical properties of rock and stress condition.

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