Geogenic radon as geophysical/geochemical tracer of active faults: an overview

Radon (Rn) in the environment originates from radioactive decay of 226Ra and 238U in the Earth's crust. Radon emanation from rocks mainly depends on Ra content, and of the mineral grain size; whereas its transport in the crust and overburden is governed by geophysical and geochemical parameters. Rn migrates towards the surface mainly through faults and fractures, and it is generally transported by a carrier gas that allows longer distance movements. The increase of radon emission along a fault system is mainly correlated with stress and/or strain energy accumulation, therefore it can be considered a short-term indicator of a potential earthquake. In particular, much effort has been spent to investigate earthquake predictability by using Rn as "geochemical precursor"; among geochemical precursors, Rn has provided high quality signals since fluid migration in the Earth's crust plays an important role in faulting processes and therefore in processes which may act as earthquake precursors. In the literature several cases of Rn anomalies preceding an earthquake are reported. However, the mechanism of observed radon anomalies is still poorly understood, and further investigations are needed to study the spatial variation of soil-gas radon across faults to assess the influence of fault motion and define how the strain can affect Rn emission from faults.