Spatio-temporal variations of 222Rn and 220Rn across seismogenic faults at Mount Vettore during the seismic sequence of the 24th August 2016 earthquake

Radon isotopes ( Rn) background values in the soil pore are typical for a specific lithology and depend on the local content of their parent nuclide 222 220 Rn and 238 232 U, Th and 226 Ra in the subsurface rock. In the international literature, many investigations have reported Rn isotopes anomalies significantly higher than background levels along active faults. Many evidences suggest that these radon anomalies can be linked to the stress/strain changes related to seismic activity that may force crustal fluid to migrate up, thereby altering the geochemical characteristics of the fault zone at surface before and after earthquakes. In this work, soil gas profiles of radon isotopes have been carried out across buried and exposed segments of the Mt. Vettore fault system, that originated the strong seismic sequence started on the 24 August 2016, as well as in correspondence of its antithetic fault system. The objective of the survey was to explore the mechanisms of migration and the spatial-temporal behaviour of radon isotopes in correspondence of still-degassing seismogenetic fault systems of the central Apennine. th The soil-radon anomalies appear spatially irregular due to variable permeability of the materials of the core zones and the damage zones. In the case of buried segment, the anomalies are 'double-peak' in profile with a maximum Rn anomalies (from 2-5 times the background values) located at the periphery in the lateral blocks and a minimum value at the axial part. The sharp spikes at both side of the buried segment of the Mt. Vettore fault are the result of the opening fractures proximal to the fault core that significant increase gas channelling and surface leaks. In contrast, background values occur in correspondence of the fault core, characterised by less permeable gauge material, where the outgassing rate is decreased by the closure of the fractures caused by the strain changes. 222 220 Rn and In the case of exposed segments of the fault, Rn anomalies mainly occur in the hanging wall, whereas diffuse degassing characterises the more exposed footwall of the fault, affected by the shallow conditions (i.e., potential atmospheric dilution due to weathering and to topographic contrast). 222 220 Rn and The repetition of the profile across the buried segment of the Mt. Vettore fault in the Castelluccio plain four days after the 6.5 Mw earthquake of 30 October 2017 highlighted an increase of Rn values (5-10 kBq/l) in correspondence of the same peaks of the previous profile. These results encourage the idea that space-and-time variations of soil-radon activity in the fault zones are mainly controlled by geodynamic factors (i.e., distribution, intensity and direction of the stress/strain changes) that control size, architecture and activity of a fault and thus determine both the spatial behaviour (i.e., size, geometry and amplitude) of a soil-radon anomaly, as well as the temporal variations of soil-radon activity during the periods of the activation of a fault segment.The development of a systematic mapping and/or continuous monitoring of the soil-radon anomalies across active fault systems can reveal relation with the architecture of the fault zones, and can have practical application for identification of faults buried under the sedimentary cover. Furthermore, these maps may constitute potentials of further assessments of radon hazard exposure to people.