Soil gas changes at Terre Calde di Medolla during and after 2012 seismic sequence

Several soil gas surveys were performed from 2008 to 2015 in Medolla (Northern Italy) within a farming area characterized by macroseeps, absence of vegetation and anomalous temperatures of soil to investigate the soil gas migration mechanism and verify the presence of a buried fault intersecting the macroseeps. In this work, we show results of soil gas measurements of radon and thoron activities, and helium and carbon dioxide concentrations, which have been carried out in the area struck of the 2012 seismic sequence.

We found that the seismic sequence sensibly influenced the soil gas distribution in the area. Indeed, soil gas anomalies are useful to recognize influences of surface features on natural gas migration. The study of the association of different gases with different origin and physical/chemical behaviour, the collection of a large number of samples during the dry season and the use of proper data analysis are fundamental in the comprehension of gas migration mechanism. The study of spatial distribution of soil gas anomalies can give information on the origin and processes involving deep and superficial gas species. In particular, the study of the spatial distribution of radon, often together with other soil gases, appears to be a suitable tool for identifying active tectonic structures in faulted areas.

²²²Rn and ²²⁰Rn were recorded starting from 2012, early after the mainshock of 20^th May. The May 2012 distribution map shows a broad sector of the area with anomalous values approximately aligned NW-SE. Radon vs thoron distribution data highlighted two different circulation mechanisms. After an initial perturbation of the system in May, a deep fluid migration is prevalent in September 2012. From 2013, the soil degassing returned to the main shallow origin. Over time, the anomalous high values of all the investigated species were always measured in correspondence of macroseeps supporting the hypothesis of a hidden fault. However, ²²²Rn values collected early after the mainshocks have ubiquitous distribution, likely due to perturbation of the system which enhanced the degassing of surficial layers and masked the deep contribution. The shallow and deep contributions presumably coexist for the other data, located at the intersection of the two trends. Over time ²²²Rn is better related to CO₂ concentrations than CH₄, in particular for the May 2012, 2013 and 2015 surveys (0.43 < r > 0.60) and, to a lesser degree, for Sept 2012 (r = 0.25). This relationship suggests that CO₂ likely acts as a carrier for ²²²Rn allowing it to quickly reach the surface. Although, generally, radon concentrations increase with flow, elevated mass flux due to high flows can dilute the ²²²Rn activities and its values recorded at the surface. This phenomenon could justify the slightly anomalous values in correspondence of macroseeps.

Geochemical surveys highlight the importance to carry out a discrete monitoring that can help to study 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.