Hydrogeologic processes drive deformations in the Matese massif (Southern Italian Apennines)

The Matese massif is an extensive outcrop of Apenninic Platform carbonate rocks located at the boundary between Central and Southern Apennines (Italy), extending ~74 km from NW to SE over an area of ~1600 km² and reaching a maximum height of 2050 m. Its geological history documents different phases of compressional and extensional tectonics which modeled the shape and size of faults within the massif. The present seismotectonic background belongs to the extensional style of the Central-Southern Apennine chain, with a series of NW-SE active extensional faults and occurrence of seismic activity, which reached intensities up to IX MCS.
The karst features of the Matese significantly affect the hydrology of the massif, where rainfall trends lead to large variations in the water reservoirs.
Recent papers report the presence of deformations induced by the elastic response of the loaded surface and the poroelastic properties of the ground. These two mechanisms are different: in the first the water load causes subsidence, in the second uplift. However, under anisotropic conditions, water pressure changes in poroelastic rocks can induce large horizontal deformations especially where highly fractured rocks may provide permeability for fluid flow. When the porosity is determined by systematic fractures, the medium is anisotropic and the surface deformation is mainly perpendicular to the fracture system. To study such processes, we analyzed the time series of 7 GNSS permanent stations located in the Matese area, and the seismicity, covering the 2005-2022 time interval. The GNSS time series of each station were detrended from a best-fitted linear model plus eventual steps due to instrumental changes, without modeling periodicities, obtaining three time series of residual displacements (N, E, Up) for each site. 
We also analyzed spring discharge and pluviometric data. The latter are used to compute the rainfall excess as the difference between the cumulated daily rainfall and the best-fitting straight line of the cumulated rainfall. Then, we applied an Independent Component Analysis to the GNSS data. This allowed us to extract from the time series, in a blind way, a signal very well correlated with hydrological data. This geodetic signal has a large horizontal amplitude and occurs perpendicular to the fracture orientations. This is also shown by the horizontal strain tensor estimated from the displacements associated with this signal, whose maximum extension axis reaches up 1µstrain perpendicular to the fault direction.
During wet periods, characterized by high rainfall excess and increasing values of spring discharge, we observe extensional deformation with stations moving “away” from the massif center; during dry periods a compressional deformation occurs, with stations coming back “toward” the massif. This suggests that the water stored within the massif is the driver of such geodetic signal: the larger the water pressure is, the larger the extensional deformation becomes; when the water level decreases, the water pressure is reduced and then compressional deformation occurs. 
Further studies should be done to understand if water circulation also indirectly affects the background seismicity.