Adachi, Mamoru

Ito, Takamori

Kawabe, Iwao

Kawasaki, Keisuke

Nagamine, Koichiro

Yamamoto, Koshi

A new station for seismo-geochemical observation has been installed at a 1,500 m bore-hole well of Nagashima spa in a southwestern suburb of Nagoya, central Japan. The well is located in the Yoro-Ise Bay active fault zone. Gas bubbles are separating from groundwater being pumped constantly at the well. In November 1997,we started automated monitoring of H_2,He, Ar, N_2,CH_4 and Rn in the gas bubbles by using a gas chromatograph and an α scintillation Rn monitor. Even in a short monitoring period less than one year, similar preseismic increases in the H_2 concentration have been recorded for nearby microearthquakes (M=1.6,1.9,2.0 and 2.6) and small and moderately large events (M=3.8 and 5.4) within the focal distance of 25 km. All the six events occurred in parallel with the Yoro fault several kilometers apart in the west. In particular, three microearthquakes before the M=5.4 event and the moderately large event itself accompanied conspicuous H_2 anomalies. Preseimic increases in the H_2 concentration up to 100% during a few to several weeks were followed by spike-like increases of H_2 concentration of 10-100% immediately after the respective four shocks. No comparable anomalies were seen in the gas species other than H_2. The preseismic increases in groundwater H_2 are plausibly geochemical diagnostics of the stress corrosion process preceding earthquakes. The preseismic H_2 anomalies for the two events (M=3.8 and 1.9) after the M=5.4 shock were less conspicuous than those for the other four events. The background-level of H_2 concentration also decreased significantly after the M=5.4 event. The subsurface H_2 behavior may have reflected the stress condition in the Yoro fault zone altered by the M=5.4 earthquake.国立情報学研究所で電子化したコンテンツを使用している

Seismo-geochemical observation at a deep bore-hole well of Nagashima spa in the Yoro-Ise Bay fault zone, central Japan

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