Subsurface gases have been surveyed in the fault zones around the Median Tectonic Line in western Shikoku Island. Thirty-two bubble gas samples have been collected for this purpose, and their chemical compositions have been determined gas chromatographically.The He/Ar, Ne/Ar, N2 /Ar and CH4 /Ar ratios of the bubble gas samples suggest that substantial separation of He from Ne has been revealed to interfere seriously with the He/Ar determination using carrier gas of oxygen when the He/Ne ratio of sample gas is as low as in the atmospheric air. Some ex perimental conditions to reduce the interference have been proposed.The He/Ar, Ne/Ar, N2 /Ar and CH4 /Ar ratios of the bubble gas samples suggest that substantial amounts of He and CH4 produced in subsurface rocks are added to groundwaters and that almost all Ne, Ar and N2 of subsurface gases are of the atmospheric origin. Concentrations of major and minor gas com ponents of the bubble gases are considerably variable reflecting the types of subsurface rocks. C02-rich bubble gases with relatively high CH4/Ar ratios are commonly observed in the Sambagawa schist zone, and their He/Ar and CH4/Ar ratios are highly fluctuating according to unsteady discharges of C02-rich gases containing He and CH4 from underground. N2-rich bubble gases with relatively low CH4/Ar ratios are predominant in the Ryoke granite zone. Some of their He/Ar ratios are fifty to ninety times as high as the atmospheric ratio. The intimate relationship between the bubble gas compositions and the types of subsurface rocks may encourage seismo-geochemical studies of subsurface gases and may suggest that CH4/Ar as well as He/Ar is a possible seismo-geochemical parameter in this region.
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