Soil H-2 and CO2 surveys at several active faults in Japan

Soil H-2 and CO2 surveys were carried out along seven active faults and around the aftershock region of the 2000 Tottori-ken Seibu earthquake in Japan. Diffuse CO2 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 CO2/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 CO2/H-2 ratios as was observed in soil gas from the fracture zone. A few days after the 2000 Tottori-kei Seibu earthquake, no CO2 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.