Pore-pressure migration along a normal-fault system resolved by time-repeated seismic tomography

In this study, we outline the space and time variations of body-wave velocities during one of the best-documented series of normal-faulting earthquakes, the 1997 Umbria-Marche sequence in central Italy. We show the fi rst ever observations of rock fracturing and fl uid overpressure propagation along a fault system by using space-time resolved variations of Vp/Vs anomalies (four-dimensional variations) that accompany earthquake migration and precede large aftershocks. The Vp/Vs increase observed before the mid-October earthquakes was related to a pore-pressure increase on fl uid-fi lled cracks in the volume around the fault. We also document that such variations are measurable only by using S- and P-wave arrival times.