Abstract
The 2003 Tokachi-oki Earthquake induced natural gas to gush out at several points in the Izumisato area of Chitose city, central Hokkaido, on September 26th. Four gas samples were taken on October 2nd and conspicuous gas seeping continued for a maximum of a month. Chemical and isotopic compositions of the gas revealed that methane was produced by CO2-reductive/hydrogenophiric methanogen and there were two different origins.
Three samples had a N2-rich, CO2-depleted composition, and its methane was relatively depleted in 13C. Enrichment in N2 implies a non-marine source, but the thickness of the late Pleistocene Higashichitose formation around Izumisato is only about 20m, which is inadequate for explaining the discharged gas volume. Therefore the gas might be generated in the Higashichitose formation beneath the Ishikari lowland, and migrated upward through the strata and along the underlying uncorformity. Some gas was emitted through Izumisato fault, but the rest of the gas accumulated at a shallow depth in the eastern area from the fault, and was quickly discharged by the earthquake. Dissolution of CO2 in the formation water through these processes resulted in a low CO2 content in the gas.
The other sample had a completely different compositions than the three samples. The gas was supposed to be generated in the marine Neogene Oiwake formation and migrated through or near the fault. This seepage seemed to be continuous and not related with the earthquake.
