Hydrogen Gas Emissions from Active Faults and Identification of Flow Pathway in a Fault Zone

It has been observed that hydrogen gas emissions from the subsurface along active faults exceed atmospheric concentrations (e.g. Sugisaki et. al., 1983). 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 (e.g. Kita et al., 1982). Based on such research, we are studying an investigation method for an assessment of fault activity using hydrogen gas emissions from fracture zones. To start, we have devised portable equipment for rapid and simple in situ measurement of hydrogen gas emissions (Shimada et al., 2008). The key component of this equipment is a commercially available and compact hydrogen gas sensor with an integral data logger operable at atmospheric pressure. In the field, we have drilled shallow boreholes into incohesive fault rocks to depths ranging from 15 to 45 cm using a hand-operated drill with a 9mm drill-bit. Then, we have measured the hydrogen gas concentrations in emissions from active faults such as: the western part of the Atotsugawa fault zone, the Atera fault zone and the Neodani fault in central Japan; the Yamasaki fault zone in southwest Japan; and the Yamagata fault zone in northeast Japan. In addition, we have investigated the hydrogen gas concentrations in emissions from other major geological features such as tectonic lines: the Butsuzo Tectonic Line in the eastern Kii Peninsula and the Atokura Nappe in the Northeastern Kanto Mountains. As a result of the investigations, hydrogen gas concentration in emissions from the active faults was measured to be in the approximate range from 6,000 ppm to 26,000 ppm in two to three hours after drilling. A tendency for high concentrations of hydrogen gas in active faults was recognized, in contrast with low concentrations in emissions from tectonic lines that were observed to be in the range from 730 ppm to 2,000 ppm. It is inferred that the hydrogen gas migrates to ground surface along fractures associated with groundwater flow. Therefore, it will be possible to estimate the groundwater flow pathways from deep underground in fracture zones around a fault by measurement of the hydrogen gas. From this standpoint, we have obtained multipoint hydrogen gas measurements across an exposed fault zone in the Atera Fault System, an active, major strike-slip fault in Central Japan and provide a continuous cross-section from fault core to damage zone. The distribution of hydrogen gas emissions, corresponding to the microscopic structure of fracture zones, have shown that large volumes of hydrogen gas emission occur where open micro-fractures are dominant and emissions were not observed in the central part of faults with abundant clay minerals. Using these simple methods, we have obtained information on the qualitative permeability of fracture zones. A rapid evaluation of the spatial heterogeneity of hydrogen gas emissions along the faults probably increase knowledge of hydrogeological structure around faults. Reference Sugisaki et al., 1983, Jour. Geol. 91, 239-258. Kita et al., 1982, JGR 87, 10789-10795. Shimada et al., 2008, Resource Geol. 58, 196-202.