Coulomb stress changes caused by repeated normal faulting earthquakes during the 1997 Umbria-Marche (central Italy) seismic sequence

Abstract

We investigate fault interaction through elastic stress transfer among a sequence of moderate-magnitude main shocks (5 < Mw < 6) which ruptured distinct normal fault segments during a seismic sequence in the Umbria-Marche region (central Apennines). We also model the spatial pattern of aftershocks and thier faulting mechanisms through Coulomb stress changes. We compute stress perturbations caused by earthquake dislocations in a homogeneous half-space. Our modeling result show that seven out of eight main shocks of the sequence occur in areas of enhanced Coulomb stress, implying that elastic stress transfer may have promoted the occurence of these moderate-magnitude events. Our Modelling results show that stress changes caused by normal faulting events reactivated and inverted the slip of a secondary N-S trending strike-slip fault inherited from compressional tectonics in its shallowest part (1-3 km). Of the 1517 available afterschocks, 82% are located in areas of positive stress changes for optimally oriented planes (OOPs) for Coulomb failure. However, only 45% of the 322 available fault plane solutions computed from polarity data is consistent with corresponding focal mechanisms associated with the OOPs. The comparison does not improve if we compute the optimally oriented planes for Coulomb failure by fixing the strike orientation of OOPs using information derived from structural geology. Our interpretation of these modeling results is that elastic stress transfer alone cannot jointly explain the aftershock spatial distribution and their focal mechanisms. Copyright 2005 by the American Geophysical Union.