Gases occurring in fracture zones of active faults were geochemically examined and the relationship between gas quality and fault activity was discussed. Gases charged mainly within fault gouges are characterized by a high cocentration of H2 and CO2. A predominant gas species—H2 or CO2—is related to the lithofacies which the fault cuts. Field observation showed that CO2 tends to occur in sediments and their related rocks, whereas H2 is usually found in igneous rocks and siliceous metamorphic rocks such as gneiss. CO2 concentration systematically varies with temperature and negatively correlated with O2 concentration. This evidence and δ 13C of CO2 (about -20‰) suggests that CO2 is biologically produced from organic materials in sediments. It is likely that, as a results of faulting, circulation of waters and soil airs along fracture zones is pomoted, and biological activity in them increases, giving rise to higher concentration of CO2 in central fracture zone. Carbon dioxide of this kind dose not stem from depths and, accordingly, we cannot expect it to be a useful precursor to earthquakes, although CO2 can be applied to the prospect of a fracture zone buried under sediments. Carbon dioxide with δ 13C of -5‰ to -17‰ in brecciated gneiss containing marble fragments may have been produced by interaction between organically derived CO2 and the marble, or alternatively may have been magmatically derived. If the CO2 is derived from depths, such deep-seated CO2 might be a useful precursor for the earthquake prediction.
