Hydrogeochemical anomalies before the 2016 Mw 6.0 and 6.5 earthquakes in Italy: new data and possible relationship with deep CO2 influx

Hydrological and hydrogeochemical changes caused by pre- and co-seismic processes have been reported for thousands of years. Transient hydrogeochemical anomalies are increasingly becoming commonly recorded before M ≥ 5.0 earthquakes at distances between 20 and more than 200 km from the earthquake epicenters. The 2016 Amatrice and Norcia earthquakes as well as the related sequence were anticipated by hydrogeochemical anomalies, in particular, since April 2016 an increase of As and V contents was recorded in groundwater from springs monitored in the Sulmona area about 70 km to the southeast of the epicentral area, and in a spring located within the epicentral area. On November 2016, these elements recovered their original very low concentrations. We considered these geochemical anomalies as reliable seismic precursors of the case-study for a dilatational tectonic setting. To validate these parameters as reliable seismic precursors and, consequently, to constitute an effective predictive tool, it is necessary to move from observed site-specific anomalies in groundwater to the comprehension of the geological and geochemical processes driving these anomalies. A pre-seismic and co-seismic elemental contribution from the bituminous-asphaltic residues and/or hydrothermal fluids could be hypothesized. However, neither additional metal (e.g. Ni, Mn) nor TDS or water isotope anomalies, which should be consequential of hydrocarbon decomposition and fluid mixing, respectively, have been detected. Consequently, a different possible source of the anomalies the steady characteristics of groundwater flow and the simultaneous increase of selected metal ions has been evaluated. Chemical and isotope models reveal that the abnormal content of As and V and the concomitant lowering of the boron isotopic ratios up to + 2‰ may be due to mineral desorption (e.g. from iron oxides and/or clays). Based on these data, the geochemical processes that led to the cited metal element increase in groundwater would be related in a new perspective on possible deep CO2 input. Furthermore, new data on the isotope ratios of the dissolved boron show potential application of the δ11B parameter as potential precursory signal of earthquakes, at least in the study area.