Abruzzo, Italy, Earthquakes of April 2009: Heterogeneous Fault-Slip Models and Stress Transfer from Accurate Inversion of ENVISAT-InSAR Data

The seismic sequence that occurred in the Abruzzo Apennines near L'Aquila (Italy) in April 2009 caused extensive damage and a large number of casualties (more than 300). The earthquake struck an area in the Italian Apennines chain where several faults, belonging to adjacent seismotectonic domains, create a complex tectonic regime resulting from the interaction among regional stress buildup, local stress changes caused by individual earthquakes, and viscous-elastic stress relaxation. Understanding such complex interaction in the Apennines can lead to a large step forward in the seismic risk mitigation in Italy. The Abruzzo earthquake has been very well recorded by interferometric synthetic aperture radar (InSAR) data, much better than the first Italian earthquake ever recorded by satellites, namely, the 1997 Umbria-Marche earthquake. ENVISAT (ENVIronmental SATellite) data for the Abruzzo earthquake are, in fact, very clear and allow an accurate reconstruction of the faulting mechanism. We present here an accurate inversion of vertical deformation data obtained by ENVISAT images, aimed to give a detailed reconstruction of the fault geometry and slip distribution. The resulting fault models are then used to compute, by a suitable theoretical model based on the elastic dislocation theory, the stress changes induced on the neighboring faults. The correlation of the subsequent mainshocks and aftershocks of the Abruzzo sequence with the volumes undergoing increasing Coulomb stress clearly evidence the triggering effect of coseismic stress changes on further seismicity. Moreover, this analysis put in evidence which seismotectonic domains have been more heavily charged by stress released by the Abruzzo mainshocks. The most important faults significantly charged by the Abruzzo sequence belong to the Sulmona and Avezzano tectonic domains. Taking into account the average regional stress buildup in the area, the positive Coulomb stress changes caused by this earthquake can be seen as anticipating the next earthquakes in the neighboring domains of 15-20 yr.