ABSTRACT

Elemental and isotopic compositions of rare gases in gas samples from “Helium spots” and CO₂-spring in the fault zone formed by the Matsushiro earthquake swarms, reported by WAKITA et al. (1978), have been analyzed mass spectrometrically. Isotopic compositions of rare gases were atmospheric except for ³He/⁴He and ⁴⁰Ar/³⁶Ar ratios. ³He/⁴He ratios were (9.12 ± 0.58) × 10^-6 and (9.10±0.69) × 10^-6, and ⁴⁰Ar/³⁴Ar ratios were 308 ± 9 and 304 ± 8 for “Helium spots” and CO₂-spring, respectively. Rare gas elemental abundance patterns for both samples were similar to a pattern produced by the dissolution equilibrium of atmospheric rare gases in water. The origin of rare gases is discussed. The ³He/⁴He ratio found in the fault zone suggests that the systematic observation of ³He/⁴He ratio may be useful for the detection of precursory phenomena of an earthquake.