Study of the transient deformation of Central and Southern Apennines from GPS observations

In the last years, geodetic measurements have shown that temporal variations in total groundwater content induce transient deformations of the crust that are mostly reflected in the vertical component. Here we present new GPS observations showing a noticeable non-tectonic transient deformation in the Apennines (Italy), mostly evident in the horizontal components, that correlate with multi-annual hydrological signals as measured by rainfall, GRACE (Gravity Recovery and Climate Experiment) and discharge of karst springs. The observed transient signal is superimposed on the long-term ~3mm/yr NE-SW directed active extension, correlates with the seasonal recharge/discharge cycle of groundwater flow and its multi-annual variations and appears to modulate the intensity of tectonic extensional opening in those regions characterized by large carbonate aquifers. In particular, periods of increasing discharge from karstic spring correlate with an enhanced extensional signal, whereas exhaustion periods correlate with reduced extensional signal. Vertical observations, being correlated with GRACE data, reflect the hydrological load variation over regional (European) scale. Horizontal deformations, conversely, affect mostly the sites located near the large carbonate massifs of the Apennines and can't be explained as load induced response. We suggest that the horizontal observed deformation is related to the presence of highly fractured shallow crust that deforms according to variable conditions of the hydraulic head within the large carbonate aquifers. We therefore propose two different models that simulates the effect of a medium permeated with sub-vertical opening fractures. These new findings could contribute to the understanding of the deformation of the large carbonate aquifers and to an improved management of these important natural resources. By focusing on the 2009 L'Aquila earthquake, we finally highlight the difficulties that arise for the estimation of interseimic trends and small tectonic signals in areas affected by such significant non-tectonic deformations.