Coseismic and initial postseismic slip of the 2009 Mw 6.3 l’Aquila earthquake, Italy, from GPS measurements

Here we report the preliminary results of GPS data inversions for coseismic and initial afterslip distributions of the MW 6.3 2009 April 6 L'Aquila earthquake. Coseismic displacements of continuous and survey-style GPS sites, show that the earthquake ruptured a planar SW-dipping normal fault with 0.6 m average slip and an estimated moment of 3.9 x 10^18 Nm. Geodetic data agree with the seismological and geological information pointing out the Paganica fault, as the causative structure of the main shock. The position of the hypocentre relative to the coseismic slip distribution supports the seismological evidence of southeast- ward rupture directivity. These results also point out that the main coseimic asperity probably ended downdip of the Paganica village at a depth of few kilometres in agrement with the small (1-10 cm) observed surface breaks. Time-dependent post seismic displacements have been modelled with an exponential function. The average value of the estimated characteristic times for near-field sites in the hanging-wall of the fault is 23.9 +- 5.4 d. The comparison between coseismic slip and post-seismic displacements for the first 60 d after the main shock, shows that afterslip occurred at the edges of the main coseismic asperity with a maximum estimated slip of 25 cm and an equivalent seismic moment of 6.5 x 10^17 Nm. The activation of the Paganica fault, spatially intermediate between the previously recognized main active fault systems, suggest that strain accumulation in the central Apennines may be simultaneosly active on distinct parallel fault systems.