Fast Determination of Moment Tensors and Rupture History: Application to the April 6th 2009, L’Aquila Earthquake

On April 6th 2009, a magnitude Mw=6.3 earthquake struck the Abruzzi region in central Italy. Despite its moderate size, the earthquake caused 293 fatalities and partially destroyed the city of L’Aquila and many villages in its surroundings. The main shock was preceded by an earthquake swarm, which started at the end of 2008. The largest earthquakes of the swarm occurred on 2009/03/30 (ML=4.1), and on 2009/04/05 (ML=3.9). To date, almost 7,000 aftershocks with ML>1.5 have been recorded by the INGV seismic network and three featured ML larger than 5.0. In this study, we present the results of the fast source parameters determination procedure adopted at the Istituto Nazionale di Geofisica e Vulcanologia (INGV) using the 2009 L’Aquila earthquake as a case study. The main task of this procedure is the fast calculation of source parameters within the first 24 hours after an earthquake. We apply a time domain moment tensor (TDMT) technique to compute the focal mechanisms of all the ML≥ 3.9 earthquakes by inverting broadband records of the Italian national seismic network. All events show normal faulting in agreement with the tectonic setting of the area. The preferred main shock moment tensor solution inferred is: strike 139°, dip 48°, rake -77° and Mw= 6.1. Using the main shock moment tensor to constrain the fault geometry, we invert the strong motion data provided by the Rete Accelerometrica Nazionale (RAN) and the MedNet station AQU to image the rupture history. The inferred model is representative of a rapid finite-fault solution to be used immediately after an earthquake to get a preliminary interpretation of ground shaking. The proposed rupture history highlights several relevant features. First, we have identified the SW dipping plane as the main shock rupture plane and the existence of rupture directivity associated with both the up-dip and SE along-strike propagation. Second, the inferred rupture velocity, constant all over the fault plane, is relatively low (2.2 km/s). Third, the rapid solution is able to identify the position on the fault plane where most of the energy is radiated, although the position and the amplitude of slip patches are not well constrained.