Shock wave radon surface signals associated with the upsurge of T-P solitons in volcanic systems

The present paper introduces a theoretical model in order to describe ground surface anomalies of radon emissions as a volcano is perturbed from its quiescent (steady) state. Based upon modern thermo-poro-elasticity theory which has been found to successfully explain rock deformation-fracturing, these anomalies are considered as being due to the build up of a buried thermo-mechanical source which in turn gives rise to the upsurge of a thermo-mechanical (or T-P) soliton. This sort of fluid solitary wave, travelling upwards through induced deformed–fractured horizons, locally perturbes a pre-existing stationary radon concentration profile as the wave approaches the ground surface. In order to model radon anomalies, a diffusive–convective transport equation of radon is adopted with Darcyʹs velocity change controlled by the upsurge of the T-P soliton. As a result, a new flow mechanism of radon emission is found, which we call shock wave radon propagation. Some cases of volcanic eruptions which have been preceded and accompanied by anomalies in radon emissions are examined in the light of the model. A reasonably good agreement in the pattern between the observed radon concentration changes and the theoretical curves is found. A theoretical tool for volcanic surveillance is thus proposed, corresponding to both an effort to shed more light on the correlations that may exist between geophysics and geochemistry of fluid emissions and the recognition, through ground surface radon anomalies, of subsurface thermo-mechanical instabilities possibly preceding volcanic eruptions.