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

On April 6, 2009, the Central Apennines were hit by a Mw=6.3 earthquake. The region had been shaken since October 2008 by seismic activity that culminated in two foreshocks with Mw>4, one week and a few hours before the main shock. We computed seismic moment tensors for 26 events with Mw between 3.9 and 5.5, using the Regional Centroid Moment Tensor (RCMT) scheme. Most of these source parameters have been computed within one hour 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. These considerations and other analyses have been possible for the rapid availability of these robust moment tensors.