Physical models related to radon emission in connection with dynamic manifestations in the upper terrestrial crust: A review

Radon emissions that are enhanced by forthcoming geophysical events as earthquakes or volcanic activity have been observed all over the World. Many of these observations have been carried out with equipments using Solid State Nuclear Tracks Detectors as sensors. However, such radon "anomalies" are not one-to-one events and the technique has not yet reached the necessary level of confidence. In the mean time, the data collected by scientists are casting some light on the involved phenomena. Radon emissions are now recognized as a rather complicated set of successive steps involving many aspects of geophysics and chemistry. Several models have been proposed as an explanation of the experimental field data. They are reviewed in this paper; ranging from radon outgassing related to fumaroles to radon emission related to the particular behaviour of aquifers in volcanic areas.