Geochemical variations during the 2012 Emilia seismic sequence

Vienna

Several geochemical surveys (soil gas and shallow water) were performed in the Modena province (Massa
Finalese, Finale Emilia, Medolla and S. Felice sul Panaro), during 2006-2014 period. In May-June 2012, a seismic
sequence (main shocks of ML 5.9 and 5.8) was occurred closely to the investigated area. In this area 300 CO2
and CH4 fluxes measurements, 150 soil gas concentrations (He, H2, CO2, CH4 and C2H6), 30 shallow waters
and their isotopic analyses ( 13C– CH4, D– CH4 and 13C– CO2) were performed in April-May 2006, October
and December 2008, repeated in May and September 2012, June 2013 and July 2014 afterwards the 2012 Emilia
seismic sequences.
Chemical composition of soil gas are dominated by CH4 in the southern part by CO2 in the northern part. Very
anomalous fluxes and concentrations are recorded in spot areas; elsewhere CO2 and CH4 values are very low,
within the typical range of vegetative and of organic exhalation of the cultivated soil.
After the seismic sequence the CH4 and CO2 fluxes are increased of one order of magnitude in the spotty areas,
whereas in the surrounding area the values are within the background. On the contrary, CH4 concentration
decrease (40%v/v in the 2012 surveys) and CO2 concentration increase until to 12.7%v/v (2013 survey). Isotopic
gas analysis were carried out only on samples with anomalous values. Pre-seismic data hint a thermogenic origin
of CH4 probably linked to leakage from a deep source in the Medolla area. Conversely, 2012/2013 isotopic data
indicate a typical biogenic origin (i.e. microbial hydrocarbon production) of the CH4, as recognized elsewhere
in the Po Plain and surroundings. The 13C-CO2 value suggests a prevalent shallow origin of CO2 (i.e. organic
and/or soil-derived) probably related to anaerobic oxidation of heavy hydrocarbons.
Water samples, collected from domestic, industrial and hydrocarbons exploration wells, allowed us to recognize
different families of waters. Waters are meteoric in origin and, apart one sample, are not thermally anomalous.
Stable isotopes of H and O point out the absence of mixing with connate waters, prolonged interaction with the
host-rock at high temperature and/or heavy gas-water exchange at depth. Isotopic carbon composition emphasizes
its organic (i.e. shallow) origin; only “La Canonica” site, the deepest well sampled in this study, shows a probable
deep(er) provenance of dissolved carbon. Waters trend away from the atmospheric end-member composition,
dissolving CO2 or CH4 depending on their redox state. Dissolved radon activity is very low, likely due to the
particular hydrogeological setting of the study area (i.e. the presence of waters with long residence times in the
considered aquifers).
Obtained results highlight a different behavior before and after the seismic events, proved also by the different
carbon isotopic signature of CH4. These variations could be produced by increasing of bacterial (e.g. peat strata)
and methanogenic fermentation processes in the first meters of the soil.