Temporal variation of seismic velocity and anisotropy before the 2009 MW 6.3 L’Aquila earthquake, Italy

Abstract

We describe a set of seismological observations on the foreshock sequence preceding the April 6th 2009, Mw 6.3, L’Aquila earthquake. The dense configuration of the seismic network in the epicenter area and the occurrence of a long foreshock sequence provide the opportunity for a detailed reconstruction of the preparatory phase of the main shock. Approaching the earthquake, we observe clear variations of the seismic wave propagation properties. The elastic properties of rocks in the fault region undergo a sharp change about a week before the earthquake. From our observations we infer that a complex sequence of dilatancy-diffusion processes takes place and that fluids play a key role in the fault failure process.