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

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

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.