Soil-gas radon anomalies in three study areas of central-northern Calabria (southern Italy)

Soil-gas radon concentrations and exhalation rates have generally been observed to be anomalously high along active faults in many parts of the world. The soil-gas method is based on the principle that faults and fractures in rocks are highly-permeable pathways along which gases can migrate upward from deep crust and mantle to soil cover, retaining their source signatures. The present study summarizes the influence of fault zones on anomalous radon concentrations in soil by integrated geophysical and geo-structural analyses in three study areas of Central-Northern Calabria (Southern Italy). Soil-gas radon surveys have been carried out by means of an alpha scintillation counting system, at 12,509 locations between 2002 and 2004. A geostatistical approach has been used to estimate the spatial distribution of soil radon concentrations. Relations among soil-gas distribution and geo-structural features have been evaluated by ordinary multi-Gaussian kriging. Highest soil radon concentrations (ca. 90 kBq m-3) have been measured in the Rossanese sector. In the three study areas, no appreciable differences can be noticed among lithotypes, with the highest concentration values (ca. 89 kBq m-3) measured in alluvial deposit and in clay. Measurements of soil-gas radon reveal anomalies clearly connected to the tectonic structures. Increased signals are linearly distributed along regional WNW-ESE trending shear zones, with main pathways of concentration also recognizable along the E-W fault system in the Rossanese sector, the N-S fault system in the Crati Graben and the Catanzaro Trough, and the NE-SW fault system in the Catanzaro Trough. The distribution of epicentres of historical earthquakes occurred between 1184 and 2001 confirms the recent activity of the same fault systems. Soil-gas radon concentrations generally increase, as expected, with decreasing distance to the faults.