Looking at fault reactivation matching structural geology and seismological data

Abstract

We investigate fault reactivation that occurred during the 1997 Colfiorito seismic sequence (central Italy), matching detailed structural geology, precise earthquake locations and mechanical models of fault interaction. The Colfiorito area, within the Northern Apennines, is characterised by a relatively recent inversion of the tectonic stress field from compression (late Miocene), to extension (late Pliocene–Quaternary). In September–October 1997 the area experienced a protracted seismic sequence consisting of six moderate magnitude earthquakes (5<Mw<6), that ruptured SW-dipping normal fault segments. The two main ruptures of the sequence show opposite directions of rupture propagation and are segmented by a strike-slip fault inherited from the compressional tectonic phase. Late in the sequence this strike-slip fault nucleated an Mw=4.3 event and an associated aftershock sequence, which we present as an example of reversal of fault slip direction. Plotting the strike-slip events on a detailed geological map, the left-lateral strike-slip fault imaged by aftershock distribution and focal mechanisms, strictly corresponds to the mapped N10° right-lateral strike-slip structure inherited from the compressional tectonic phase. The two different data-sets for this strike-slip fault show the same geometry but opposite kinematics. We analyse the coseismic elastic static stress changes due to the occurrence of the normal fault events in order to investigate the mechanical coupling and interaction between the NW–SE oriented normal fault system and the intervening N10° inherited fault. We show that the elastic stress perturbation increased on the shallow portion of the activated volume where the N10° structure is located, and promoted its reactivation with a strike-slip motion. The different amount of slip experienced along the two normal faults responsible for the greater mainshocks favoured the left-lateral strike-slip kinematics.