Helium isotopes as a tool for detecting concealed active faults

A magnitude ( M j ) 7.3 crustal earthquake occurred in the western Tottori area, southwest Japan, on 6 October 2000. However, there was no obvious prefaulting indication at surface of a fault corresponding to the western Tottori earthquake in 2000. This study was undertaken to elucidate the geographic distribution of 3 He/ 4 He ratios around the seismic source region using helium isotope data obtained from groundwater samples from drinking water wells. The maximum 3 He/ 4 He ratio observed was from the water well nearest to the epicenter of the main shock. In addition, there appears to be a clear trend of decreasing 3 He/ 4 He ratios with distance away from the main trace of the probable fault segments. The observations provide significant evidence that the source fault of the earthquake in 2000 is associated with leakage of mantle volatiles through the crust to the Earth's surface. We suggest that helium isotopes can be regarded as a tool for investigating and/or mapping concealed active faults with no surface expression.