Claudio Chiarabba, Pasquale De Gori, Enzo Boschi; Pore-pressure migration along a normal-fault system resolved by time-repeated seismic tomography. Geology 2009; 37 (1): 67-70. doi: 10.1130/G25220A.1
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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 first ever observations of rock fracturing and fluid 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 fluid-filled cracks in the volume around the fault. We also document that such variations are measurable only by using S-and P-wave arrival times.