The detection and interpretation of hydrogen in fault gases

Hydrogen gas can be released by chemical and mechanical changes in crustal rocks. Once released, it is highly mobile, buoyant, and almost insoluble in groundwater. A fault system may act as a conduit, allowing hydrogen to accumulate in soil gases near a surface expression. Since hydrogen is scarce in ambient air, its presence at elevated levels in soil gases may be a tool for fault mapping. In order to evaluate this tool, we surveyed eleven different faults by measuring the concentration of hydrogen and methane in 2 to 21 soil-gas samples that were collected near each of them. The sense of motion at four of those faults is normal (western United States, Greece), at five it is strike-slip or dip-slip (California, Colorado, Japan), and at two it is thrusting (California). At four of these faults (Hebgen Lake, Yellowstone, Yamasaki, Burro Mountain) maximum concentrations of hydrogen ranged from 80 ppm to 70% and methane from 300 ppm to 5%. All other sites showed ambient levels of both gases, except for one sample taken at Mt. Borah, Idaho, that was 2% methane. From this preliminary study it is not clear whether the presence of hydrogen is correlated uniquely to the location of faults or whether it occurs randomly. The conditions required to produced and accumulate hydrogen are also not clear. Excess hydrogen may well be produced by different mechanisms in different geological regimes. For example, if ferrous hydroxide is present in local rocks, it may react to produce hydrogen. Detailed and extensive studies are needed to clarify the connection between hydrogen and tectonic faulting.