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Geographic distribution of $$^{3}$$He/$$^{4}$$He ratios along seismic source faults in Japan

日本列島における震源断層周辺のヘリウム同位体比の分布

梅田 浩司; 浅森 浩一; 草野 友宏

Umeda, Koji; Asamori, Koichi; Kusano, Tomohiro

内陸地震を引き起こしたと考えられる断層周辺の地下水の溶存ガスや遊離ガス中に含まれる希ガス同位体の測定を行い、断層周辺のヘリウム同位体比の特徴を検討した。その結果、平均変位速度が大きい断層周辺ほど高い$$^{3}$$He/$$^{4}$$He比が認めらることから、活動性が高い断層ほど断層を通じたマントル起源ヘリウムの上昇が生じやすいと考えられる。断層ガスの同位体組成は、変動地形の明瞭でない断層の調査技術や断層の活動性の評価技術に有効であることを示唆する。

It is well known that mantle degassing does not occur homogeneously over the Earth's surface. This study was undertaken to elucidate the geographic distribution of $$^{3}$$He/$$^{4}$$He ratios around seismic source faults in Japan, using helium isotope data obtained from gas samples. Several case studies suggest that there is a significant trend of high $$^{3}$$He emanations along the trace of active faults, resulting in leakage of mantle volatiles through crustal pathways (faults) due to more frequent development of higher permeability pathways and/or upwelling of mantle fluids through the ductile lower crust. From the viewpoint of site selection and implementation of a geological disposal facility, helium isotopes may be regarded as a tool for investigating and/or mapping concealed active faults with no surface expression.

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