Origin of Hydrogen and Carbon Dioxide in Fault Gases and Its Relation to Fault Activity

Gases occluded in fracture zones of active faults are characterized by a high concentration of H₂ and/or CO₂. A predominant gas species-H₂ or CO₂-is related to the lithofacies which the fault cuts. Carbon dioxide concentration in sediments fluctuates with temperature. This evidence and σ ¹³C ofCO₂ (about -20‰) suggests that CO₂ originates from organic materials. Carbon dioxide with σ ¹³C of 5‰ to - 17‰ in brecciated gneiss containing marble may have been produced by interaction between organically derived CO₂ and the marble, or alternatively, may have been magmatically derived. Hydrogen usually occurs in sheared silicate rocks, and its concentration fluctuates a great deal. The concentration of H₂ from active faults associated with historical earthquakes usually amounts to as high as several percent in maximum, whereas the concentration of H₂ from Quaternary faults not associated with historical earthquakes is at most 100 ppm. Laboratory experiments showed that much H₂ is generated from paste made of newly pulverized rocks and water, suggesting that the fresh mineral surface formed by tectonic stresses reacts with groundwater to produce H₂. Since the mineral surface loses its activity with time, discrimination between recently moved faults and other Quaternary faults can be made by the H₂ concentration. Hydrogen isotope thermometer, as well as field evidence, suggests a deep seated origin of H₂ in an active fault. Hydrogen measurements at monitoring stations offer information at depth on mechanisms that operate prior to earthquakes.