Research into the study of the interactions between the Gran Sasso aquifer and seismic phenomena, conducted by the INGV National Institute of Geophysics and Volcanology in collaboration with the Gran Sasso National Laboratories of the INFN National Institute of Nuclear Physics and with the Department of Civil, Building-Architecture and Environmental Engineering (DICEAA) of the University of L'Aquila, has detected variations of some physical parameters of the groundwater of the Gran Sasso massif, in conjunction with the seismic event that hit Amatrice in August of 2016.
The study entitled "A record of changes in the Gran Sasso groundwater before, during and after the 2016 Amatrice earthquake, central Italy" by Gaetano De Luca, Giuseppe Di Carlo and Marco Tallini, recently published in the journal Scientific Reports of Nature (LINK), is based on continuous high-sampling measurements (20 measurements per second) of the hydraulic pressure, temperature and electrical conductivity of the water, measurements made, starting from May 2015, on a horizontal survey, called S13, carried out at the end from the 80s during the excavation works, and located near the motorway tunnel and the INFN Laboratories of the Gran Sasso.
The presence of the S13 horizontal boring offered the unique opportunity to investigate the deepest part of the Gran Sasso calcareous aquifer, located in the seismically active zone of the central Apennines. This area of the central Apennines is also monitored by a rather dense regional seismic network together with the national INGV seismic network.
The data acquired, starting from May 2015, showed clear and interesting signals in the hydraulic pressure and electrical conductivity of the Gran Sasso aquifers before, during and after the earthquake that occurred on August 24, 2016 (01:36:32 UT) with epicenter about 39 km from the study site. As part of the measurements performed, the researchers therefore focused the analysis on the data relating to these signals, which showed anomalies in the hydraulic pressure starting from August 19, 2016, i.e. five days before the event, large and asymmetrical fluctuations : negative micropulses, which had not been detected in the previous data and which continued until the end of August 2016.
"Obvious hydrogeochemical and hydrogeological changes were observed, before and during the seismic sequence of Amatrice, including over 95.000 events to date", explains Gaetano De Luca, researcher at the National Institute of Geophysics and Volcanology. “The interpretation of these micro pulsations – continues De Luca – is most likely to be found in the arrival of large quantities of gas, presumably carbon dioxide, in the form of bubbles”. The detected anomalies could be classified both as short-term seismic precursors, in the case of the hydraulic pressure anomalies detected 5 days before the Amatrice earthquake, and as long-term seismic precursors, in the case of the anomalies in the hydraulic pressure and conductivity signals. electricity, recorded about 40-60 days before the event: careful analysis has, in fact, allowed us to detect the beginning of a slight change in pressure up to 40 days before the Amatrice earthquake, and a significant variation in the electrical conductivity about 60 days earlier.
"These data represent the first observations, in conjunction with seismic events, of signals in the hydraulic pressure, measured with continuous monitoring at high sampling frequency of a borehole positioned inside the Gran Sasso stratum", explains Giuseppe Di Carlo, researcher of the Gran Sasso National Laboratories. "It is possible - continues Di Carlo - that the variations in groundwater respond to deep crustal deformation processes that occur long before significant earthquakes occur". Several mechanisms have been proposed to explain these variations during the seismic cycle, including the "dilatancy theory" which assumes that, during the pre-seismic phase, i.e. before failure, the increase in rock volume is favored by micro-fracturing inside the point of origin of the earthquake. Micro-fracturing also induces an increase in the permeability of rocks, favoring changes in groundwater and the movement of geogas.
“The groundwater monitoring in the S13 borehole revealed the existence of highly dynamic aquifer behaviour,” explains Marco Tallini of the University of L'Aquila. "The data collected during these twenty-one months have proved to be a fairly useful tool for characterizing the scenario in groundwater monitoring and more clearly identifying the anomalies observed and their possible connection with phenomena related to active tectonics and hydrogeology", concludes Tallini .
“We interpret the presence of negative micropulses in the hydraulic pressure data – explains De Luca – as a result of a possible movement from the depths of geogases, mainly CO2, coherently with recent studies suggesting a link between fault mechanics, seismicity, dynamics of underground fluids and upwelling from the mantle of CO2 ".
Now, further insights into the relationship between earthquakes and changes in groundwater parameters in the vicinity of large seismogenic faults are needed for a full understanding of pre-seismic, co-seismic and post-seismic processes.