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

Trique, M; Richon, P; Perrier, F; Avouac, JP; Sabroux, JC. NATURE, 399: (6732) 137-141.

Two of the most often cited earthquake precursors are radon emanation and electric potential variations(1-6), but these few reported examples have generally been deemed questionable(7-11). If a mechanism relating crustal deformation to radon emanation or electrical signals does indeed exist, it is thought to involve fluids(12-19). Some preliminary insight has been gained into these processes from the study of natural systems under controlled mechanical and hydrological conditions(20). Here we report electric potential variations, radon emanation and deformation measurements recorded since 1995 in the French Alps in the vicinity of two artificial lakes which have strong seasonal variations in water level of more than 50 metres. We observe that electric potential variations and radon emanations are repeatedly associated 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 accelerating 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 transport properties to deformation.