Reconstruction of Fault Geometry Through Hypocenter Clustering for Coulomb Stress Analysis During the L'Aquila Earthquake Swarm.

The 2009 L'Aquila seismic sequence in central Italy occurred on an approximately 50 km long south-west dipping normal fault system. The 6.1 M_Wmainshock was preceded by several months of persistent foreshocks of M_W < 4.1. Aftershocks continued for over a year and included 5 M_W > 5 events. Fluid diffusion and dilatancy have been hypothesized to be major factors in the development of these foreshocks and the rest of the sequence. Over 50,000 aftershocks within the swarm have been relocated, and 4000 of their focal mechanisms have been identified. By combining these data with GPS measured displacements during the mainshock, we identified the (i) mainshock geometry, (ii) mainshock slip distribution, and (iii) Coulomb stress changes experienced by the aftershocks. We inverted for mainshock models by minimizing the calculated displacement error at several coseismically recorded GPS stations. Stress and strain are found using finite, rectangular, analytical solutions for stress under a spline representation of slip distribution. The geometry of the faults have been reconstructed by clustering the hypocenters of the aftershocks. The Coulomb stress calculations show that including the high magnitude aftershocks increases the match between the observed and modelled stress drop of the swarm of aftershocks. Our models nevertheless display many erroneous aftershocks with negative change in Coulomb stress on both possible planes suggesting incorporating stress error bars, geological heterogeneities, and possibly fluid mobilization along and between faults.