The large karst aquifers of the Apennines deform in relation to seasonal variations in rainfall. This is demonstrated by satellite and hydrological data, analyzed during a multidisciplinary study conducted by INGV in collaboration with the Universities of Sannio, Lyon and Acquedotto Pugliese SpA. The study was published in the Journal of Geophysical Research.
The limestone mountains of the Apennines, home to important water reserves for cities such as Rome, Naples and Bari, deform according to the amount of rain received. This was established with a study entitled Transient deformation of karst aquifers due to seasonal and multiyear groundwater variations observed by GPS in the southern Apennines, published in the Journal of Geophysical
Research (http://onlinelibrary.wiley.com/doi/10.1002/2016JB013361/full) was a group of researchers from the National Earthquake Center of the National Institute of Geophysics and Volcanology (CNT-INGV), the University of Sannio, Lyon and the Società Acquedotto Pugliese SpA. For the research, displacement measurements recorded by GPS stations were used (Global Positioning System) of the INGV RING network (http://ring.gm.ingv.it), changes in gravity measured by the GRACE satellite (http://grace.jpl.nasa.gov), rainfall series and flow measurements of the karst spring of Caposele (province of Avellino).
“At first we didn't understand the origin of the signals observed on the time series of the GPS stations”, explains Francesca Silverii, a young researcher who obtained her doctorate from INGV with a thesis on this topic at the University of Bologna. “The initial subject of the thesis was the study of post-seismic deformation following the 2009 L'Aquila earthquake, but the observed signals immediately suggested a very different origin, linked to hydrological factors”. The study showed that the deformation measured by the GPS is closely correlated with the variation of the rainfalls of the large aquifers. In particular, the large limestone masses of the Apennines, home to huge water reserves, expand and contract according to the quantity of water they receive during the seasonal recharge period which, for the Italian peninsular area, occurs during the autumn and winter rainfall and snowfall (Figure 1).
"The deformations associated with the seasonal recharge phases", says Nicola D'Agostino, INGV researcher and research coordinator, "reach about ten millimeters and overlap the slow and constant separation of about 3 mm/yr between the Tyrrhenian and Adriatic coasts . These results will help us to more accurately identify areas where tectonic deformation is accumulating and will be released in the future by earthquakes such as those observed in recent months."
An essential role was played by the considerable amount of data provided by Acquedotto Pugliese SpA, the main operator in southern Italy in the field of water supply and the Integrated Water Service. This information, collected thanks to the efforts of the working group coordinated by Gerardo Ventafridda, geologist of the Apulian Aqueduct SpA, derives from the historical propensity of the Apulian Aqueduct to the knowledge and analysis of the fundamental quantities, through which to guarantee quality and efficiency of the service done. In this sense, the availability and reliability of the collected data is guaranteed by the activity of organizational structures, which use this data in order to optimize the use of water resources and their management in case of emergencies and to contribute to the development of scientific research on the subject.
"These results are very interesting for understanding the characteristics and optimal management of the large water reserves of the Apennines and open up important research perspectives on karst aquifers", concludes Francesco Fiorillo, Professor of Applied Geology at the University of Sannio.
This year, on the occasion of the presentation of the work at the congress of the National Group of Geophysics of the Solid Earth (GNGTS) in Lecce, the first author of the research, Francesca Silverii received the Prize of the "Licio Cernobori" Association for Geophysics, conferred annually to young researchers.
Extended
We present GPS, hydrological, and GRACE (Gravity Recovery and Climate Experiment) observations in southern Apennines (Italy) pointing to a previously unnoticed response of the solid Earth to hydrological processes. Transient patterns in GPS horizontal time series near to large karst aquifers are controlled by seasonal and interannual phases of groundwater recharge/discharge of karst aquifers, modulating the extensional ∼3 mm/yr strain within the tectonically active Apennines. We suggest that transient signals are produced, below the saturation level of the aquifers and above a poorly constrained depth in the shallow crust, by time-dependent opening of subvertical, fluid-filled, conductive fractures. We ascribe this process to the immature karstification and intense tectonic fracturing, favoring slow groundwater circulation, and to multiyear variations of the water table elevation, influenced by variable seasonal recharge. The vertical component displays seasonal and multiyear signals more homogeneously distributed in space and closely correlated with estimates of total water storage from GRACE, reflecting the elastic response of the lithosphere to variations of surface water loads. The different sensitivities of vertical and horizontal components to the hydrologically induced deformation processes allow us to spatially and temporally resolve the different phases of the water cycle, from maximum hydrological loading at the surface to maximum hydrostatic pressure beneath karst aquifers. Finally, we suggest that transient deformation signals in the geodetic series of the Apennines are correlated to large-scale climatic patterns (Northern Atlantic Oscillation) through their influence on precipitation variability and trends at the regional scale.
