地震の前兆現象としての断層ガス中のHe/Ar, N₂/Ar比の変動

抄録

The utility of the ratios of He/Ar and N₂/Ar of fault gases as an earthquake precursor was discussed and examined. The ratio of helium to argon produced in the lithosphere remains rather constant (≅10) for the past several hundred million years irrespective of rock types. The ratio in the lithospheric air amounts to 10⁵ times higher than that of the atmospheric air (5.7×10^-4). The lithospheric air released by the formation of cracks and migrated by the stress in rocks prior to earthquakes, therefore, can be easily detected by means of the ratio. The N₂/Ar ratio is also expected to be a useful parameter of a lithospheric air, because the ratio in natural gases and volcanic gases is generally higher than that of the atmospheric air. These ratios do not change a dilution process by other gases such as CH₄ and CO₂, and accordingly the ratios are more useful parameter of lithospheric air than a single element. The ratios of He/Ar and N₂/Ar are gas-chromatographically measured without vacuum processes, temperature and pressure corrections and other complicated treatments.
The result of successive measurement, since 1976, of the ratios in a fault at Inuyama Crustal Movement Observatory, Nagoya University, indicated that the ratios continuously fluctuated and the peak in the variation of the ratios of He/Ar and N₂/Ar appeared prior to the most felt earthquakes in Nagoya (M≥4). The peaks of He/Ar preceeded those of N₂/Ar for all events, which might be attributed to the difference in diffusion velocity of He and N₂ through capillary cracks. The time lag of N₂/Ar peak in case of the large earthquake is longer than that of the others. These features emerged in the variation of the ratios can possibly provide a practical mean of predicting earthquakes.