GNGTS 2016 - Atti del 35° Convegno Nazionale

GNGTS 2016 S essione 1.2 239 extending on a spatial length-scale similar to the GRACE footprint (≈200,000 km 2 ). Indeed, d������ �������� ����� �������� �� ������� � ������� ������ ��� ���������� ������ �� ��� �������� espite possible local effects, we observe a similar annual and multi-year signal at GPS stations both near the main carbonate aquifers and far from them, where no significant multi-year signal in the horizontal components is observed. The significant correlation with karst spring discharge suggests that the observed horizontal ground deformation depends on a mechanical process sensitive to the level of hydrostatic pressure in the shallow crust beneath karst aquifers. The observed pattern and magnitude of the horizontal deformation require that the process responsible for the hydrologically-related deformation of the carbonate aquifers should be able to produce an anisotropic horizontal deformation with a preferential NE-SW expansion direction and a marginally significant vertical deformation. This response could be due to crust anisotropy caused by preferentially aligned fractures and micro-cracks (e.g., Pastori et al., 2009). We thus ascribe the observed horizontal signal to the response of a sub-vertical, fluid-filled, conductive system of fractures to the water table height variations. In order to simulate the observed process we propose a simple two-parameters model of tensile dislocations, which are opening in response to variations of hydrostatic pressure (Fig. 2). We constrain the time-dependent opening of these dislocations to be linearly related to the discharge of the Caposele spring, whose time variations are considered to closely reflect the regional hydrological signal. This simple first-order approach used to model the observed transient signals (single tensile dislocations beneath the main karst systems) reflects the current limited knowledge of the hydraulic properties (fracture densities, rheology, poroelastic parameters) of the shallow crust, but shows a remarkable first-order agreement between the model and the observations (Fig. 2). The results of this work reveal a strong source of transient horizontal deformation in the shallow crust and highlight a new and accurate tool, the GPS data, which can be used to study the deformation and behavior of the carbonate aquifers of the Apennines, which represent a strongly important resource for the Italian territory. 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