Present kinematics of Central and Northern Italy from continuous GPS measurements

The geodetic data acquired in the last 10 years by a fairly dense network of permanent GPS stations located in Central and Northern Italy are used to reconstruct the present horizontal velocity field in the study area and to evaluate the coseismic and postseismic effects of the recent L’Aquila earthquake (April 6, 2009 Mw = 6.3). The distribution of geodetic velocities indicates that during the observation period the outer Adriatic sector of the Central and Northern Apennines has moved faster than the surrounding zones. It is pointed out that this short term kinematic behaviour is compatible with the long term kinematics indicated by the post-early Pleistocene deformation pattern observed in the central Mediterranean area. The short term kinematic pattern in the Central Apennines has been perturbed by the L’Aquila earthquake. The daily position time series of the sites located within 140 km from the epicentre show a coseismic offset followed by a non linear time dependent displacement which gradually vanishes in 1–3 months, after which the trend of daily positions seems to become linear again. The non linear transient phase has been modelled by a decaying exponential function, possibly connected with postseismic viscoelastic relaxation. The displacements developed during this last phase are rather different from coseismic displacements. Also, the velocities tentatively deduced from the postseismic linear trends (observed for about 17 months) show quite different orientations with respect to both coseismic and short term postseismic displacements. This evidence suggests that the tectonic processes which developed in the Central Apennines after the L’Aquila event were complex, possibly related to shallow afterslip, deeper aseismic slip and viscoelastic relaxation of the lower crust and/or uppermost mantle.