Abstract
The 3He/4He and 4He/20Ne ratios of gases emergent in the southwestern part of Japan have been measured by a mass spectrometer. The geographical distribution pattern of the 3He/4He ratios in the Kinki district, a non volcanic, forearc region where microseismic activity at shallow depths has continued over a long period of time, is strikingly different from those found in the rest of the Japanese Islands, including the Shikoku district which lies next to Kinki. Anomalously high 3He/4He ratios, up to 9.69 × 10−6, are found in the Kinki district. All the observed phenomena (high emanation of 3He of magmatic origin, high‐temperature hot springs, high terrestrial heat flow, hypocentral distribution of microearthquakes, a local irregularity in the dip direction of the subducting Philippine Sea plate, and the presence of molten materials inferred from the characteristic phases of SxP and SxS reflected waves in microseismograms) strongly suggest the presence of a shallow magma body beneath the area.
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