Radon emanation and electric potential variations associated with transient deformation near reservoir lakes

Two of the most often cited earthquake precursors are radon emanation and e lectric potential variations(1-6), but these few reported examples have gen erally been deemed questionable(7-11). If a mechanism relating crustal defo rmation to radon emanation or electrical signals does indeed exist, it is t hought to involve fluids(12-19). Some preliminary insight has been gained i nto these processes from the study of natural systems under controlled mech anical and hydrological conditions(20). Here we report electric potential v ariations, radon emanation and deformation measurements recorded since 1995 in the French Alps in the vicinity of two artificial lakes which have stro ng seasonal variations in water level of more than 50 metres. We observe th at electric potential variations and radon emanations are repeatedly associ ated with transient deformation events induced by variations in lake levels . These events are characterized by a change in ground tilt which deviates from the expected elastic response, and are associated with periods of acce lerating strain, which suggests that accelerated loading can enhance fluid transport properties. Qualitatively, this behaviour can be accounted for by a model in which straining induces fluid overpressure and dynamic flow in cracks. These observations may shed light on the sensitivity of rock transp ort properties to deformation.