新第三紀珪質岩における断層の解析事例

A Case study of analysis of faults in Neogene siliceous rocks

石井 英一   ; 福島 龍朗

Ishii, Eiichi; Fukushima, Tatsuo

岩盤中には地下水の流れや物質移動に大きな影響を及ぼす割れ目がしばしば存在することから、地質構造モデルを構築する際には、これらの割れ目を十分に考慮する必要がある。そこで本研究では、水みちの可能性のある断層の調査手法の開発の一環として、新第三紀珪質岩中に分布する断層の地質学的特徴と形成過程を露頭観察・ボーリングコア観察・孔壁観察の結果から検討し、その検討結果に基づいて、断層の水理特性の評価や水理学的に重要と思われる断層を考慮した地質構造概念モデルの構築を試みた。その結果、褶曲構造が発達する新第三紀珪質岩中の充填鉱物や熱水変質の認められない断層について、(1)分類を行う際には、断層方向の褶曲軸並びに層理面との関係,変位センス、及び変位量への着目,(2)形成過程を検討する際には、断層同士の切り切られの関係の観察,褶曲運動との関係の検討、及び小断層解析による断層運動時の応力場の推定,(3)水理特性を検討する際には、地表付近における断層沿いの酸化状況,断層の分布密度,断層の形成過程、及び断層の方向と応力場との関係の検討が効果的であった。

Because some fractures might be water conducting features in rocks, it is necessary to consider sufficiently the fractures, for development of a geological model. In this study, we have tried to understand geological characteristics and formative processes of faults in Neogene siliceous rocks, based on results of outcrop observation, boring-core observation, and borehole-wall observation, in order to develop an investigation technique for water conducting features. In addition, we have tried to estimate permeability of faults and to develop a geological conceptual model including water conducting features. The results of this study suggest the follow: Concerning faults without filling minerals and hydro-thermal alteration in Neogene siliceous rocks including fold structures, (1) in order to classify the faults, an attention of relation between fault direction and fold axis/bedding direction, of displacement sense, and of displacement is effective. (2) in order to understand the formative process of the faults, an observation of crosscutting relations of faults, an analysis of relationship between faulting and folding, and an inference of stress conditions of faulting by fault striation analysis are effective. (3) in order to estimate hydro-geological property, an observation of redox front along faults near the surface, and an analysis of fault density, of formative process of faults, and of relation between fault direction and stress axis are effective.