Seismic moment tensors of the April 2009, L'Aquila (Central Italy), earthquake sequence

P>On 2009 April 6, the Central Apennines were hit by an M-w = 6.3 earthquake. The region had been shaken since 2008 October by seismic activity that culminated in two foreshocks with M-w > 4, 1 week and a few hours before the main shock. We computed seismic moment tensors for 26 events with M-w between 3.9 and 6.3, using the Regional Centroid Moment Tensor (RCMT) scheme. Most of these source parameters have been computed within 1 hr after the earthquake and rapidly revised successively. The focal mechanisms are all extensional, with a variable and sometimes significant strike-slip component. This geometry agrees with the NE-SW extensional deformation of the Apennines, known from previous seismic and geodetic observations. Events group into three clusters. Those located in the southern area have larger centroid depths and a wider distribution of T-axis directions. These differences suggest that towards south a different fault system was activated with respect to the SW-dipping normal faults beneath L'Aquila and more to the north.