GNSS data provide unexpected insights in hydrogeologic processes

The analysis of long time series of Global Navigation Satellite System (GNSS) observations has recently evidenced that the slow tectonic processes are not the only ones producing the observed slow deformations, but the Earth 's crust reacts also to stresses induced by pressure variations and water circulation. The basic mechanisms are substantially of two kinds: deformations induced by the elastic response of the loaded surface and deformations due to the poroelastic properties of the ground. These mechanisms are quite different, in the first case the water load causes subsidence, in the second uplift; both create horizontal deformations moving away from the centre of deformation. Under anisotropic conditions, water pressure changes in poroelastic soils can induce large horizontal deformations especially where highly fractured rocks may provide permeability for fluid flow. Both elastic and poroelastic phenomena are observable and measurable by continuous GNSS monitoring of ground deformations. Both can be triggered by periodical atmospheric processes but also by extreme events, like heavy rainfalls. We will show a few case studies, observed in the Italian area, that demonstrate how the deformation patterns, at different repeating periods, clearly correlate with groundwater circulation in different environmental condition.