Development of a Bayesian approach to assess the probability of concealed active faults existing using helium isotope ratios

Martin, A.*; Ishimaru, Tsuneari ; Umeda, Koji; Asamori, Koichi

In Japan, numerous studies have been carried out to assess the stability of the geological environment including in particular, the spatio-temporal distribution of active faulting in the context of site selection of a radioactive waste repository and/or assessing the safety of current nuclear facilities etc. One key concern is the existance of active faults that do not show any surface rupture. High He-3/He-4 ratios which tend to be found in volcanic regions have also been measured in non-volcanic regions. This has been attributed to degassing from the mantle with faults potentially acting as conduits. Studies carried out in the western Tottori district have shown the potential of using He/He ratios as a means of providing indirect evidence of the existence of source fault(s) that caused the 6 Oct 2000 Tottori earthquake (Mw 6.8), but which had no apparent surface indication prior to the earthquake.