Geographic distribution of He/He ratios along seismic source faults in Japan

Umeda, Koji; Asamori, Koichi; Kusano, Tomohiro

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 He/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 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.