Helium isotopes as a tool for detecting concealed active faults

Umeda, Koji; Ninomiya, Atsushi

A magnitude () 7.3 crustal earthquake occurred in western Tottori Prefecture, southwest Japan, on 6 October 2000. However, there was no indication of an earthquake surface fault corresponding to the 2000 earthquake. In order to elucidate the geographic distribution of the He/He ratios around the seismic source region, new helium isotope data for groundwater samples from drinking water wells were acquired. The maximum He/He ratio was observed from the well located nearest to the epicenter of the main shock. In addition, there appears to be a clear trend of decreasing He/He ratios with distance away from the main trace of the estimated fault segments. The observations provide significant evidence that the source fault of the 2000 earthquake 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 concealed active faults with no surface expression.