Soil H
2 and CO
2 surveys were carried out along seven active faults and around the aftershock region of the 2000 Tottori-ken Seibu earthquake
in Japan. Diffuse CO
2 effluxes were also measured along one fault and around the 2000 aftershock region. The results show highly variable H
2 concentration in space and time and it seems that the maximum H
2 concentration at each active fault correlates with fault activity as exemplified by the time of the latest big earthquakes.
Even though observed H
2 concentrations in four faults were markedly lower than those collected previously in the latter half of the 1970s, it is
evident that the higher H
2 concentrations in this study are due to the addition of the fault gases. Comparing the chemical composition of trapped gases
(H
2: 5–20% and CO
2/H
2: 0.5–12) in fractured rocks of drill cores bored at the Nojima fault, a soil gas sample with the highest H
2 concentration showed large amounts of the trapped fault gas, diluted with atmospheric component. The profile experiment across
a fracture zone at the Yamasaki fault showed higher H
2 concentrations and lower CO
2/H
2 ratios as was observed in soil gas from the fracture zone. A few days after the 2000 Tottori-kei Seibu earthquake, no CO
2 effluxes related to the occurrence of earthquakes were observed at the aftershock region. However, only above the epicenter
zone, relatively high H
2 concentrations in soil gases were observed.
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