Hydrogen gas emissions from active faults and identification of flow pathway in a fault zone

活断層における水素ガス放出と断層帯での流体移行経路の把握

石丸 恒存 ; 丹羽 正和   ; 黒澤 英樹; 島田 耕史   

Ishimaru, Tsuneari; Niwa, Masakazu; Kurosawa, Hideki; Shimada, Koji

地質環境の長期安定性を検討するうえでは、調査対象地域に分布する断層の活動性や水理特性などを把握することが必要となる。このような断層にかかわる調査技術のうち、地球化学的なアプローチとして、断層破砕帯などから放出される水素ガスを利用した調査手法の適用性の検討を進めている。本研究では、露頭において簡便に水素ガス濃度の測定を可能とする手法を考案し、我が国の幾つかの主要な活断層と地質断層を対象に水素ガス測定を行った。その結果、活断層では地質断層に比べて水素ガス放出量が多い傾向が認められた。さらに、断層帯での水素ガスなどの流体のフラックスを把握することは、主要な流体移行経路の把握にも寄与すると期待できることから、代表的な横ずれ活断層である阿寺断層の連続露頭を対象に多地点での水素ガス測定を行った。その結果、開口性割れ目の発達する地点で水素ガス放出量が多いことがわかり、本手法が相対的な透水性の大小の把握にも有効である見通しを得た。

It has been observed that hydrogen gas emissions from the subsurface along active faults exceed atmospheric concentrations. Experimental studies have shown that hydrogen gas is generated in a radical reaction of water with fractured silicate minerals due to rock fracturing caused by fault movement. Based on such research, we are investigating the applicability of a technique using hydrogen gas emissions from fracture zones. To start, we devised portable equipment for rapid and simple in situ measurement of hydrogen gas emissions. Then, we measured the hydrogen gas concentrations in emissions from major active faults and other geological features in Japan. As a result of the investigations, a tendency for high concentrations of hydrogen gas in active faults was recognized, in contrast with low hydrogen gas concentrations in emissions from geological features. It is inferred that the hydrogen gas migrates to ground surface along fractures associated with groundwater flow. Therefore, it is expected that by measurement of the hydrogen gas it will be possible to estimate hydraulic properties such as the continuity of groundwater pathways in fracture zones around a fault. Based on this understanding, we obtained multipoint hydrogen gas measurements across an exposed fault zone in the Atera Fault System, and provide a continuous cross-section from fault core to damage zone. The distribution of hydrogen gas emissions showed that large volumes of hydrogen gas emission occur where open fractures are significant and no emissions were observed in the central part of faults with abundant clay minerals. Using these simple methods, we have obtained data on the relative degree of permeability of the geological structures.