GNGTS 2023 - Atti del 41° Convegno Nazionale

Session 1.1 - POSTER GNGTS 2023 A reviewed model of the 1997 Umbria-Marche seismic sequence in light of more than 20 years of new seismological and subsurface data M. Buttinelli, F.E. Maesano, M. Anselmi Istituto Nazionale di Geofisica e Vulcanologia, Rome, Italy During the 1997 Umbria-Marche sequence, six main shocks with 5 < Mw < 6 occurred (Chiaraluce et al., 2003-2004; Antonioli et al., 2005) within a seismic sequence of ~ 2000 earthquakes. All the large shocks originated on adjacent parallel NW trending normal faults whose extent varies between 5 and 10 km at a hypocentral depth of 5 –6 km (Amato et al., 1998). The Umbria-Marche sequence occurred in the Central Apennines, which is characterized by a multiphase tectonic evolution, with two main phases: an upper Miocene-lower Pliocene compressional phase forming E-NE verging thrusts and folds and a superimposed Quaternary extensional phase which follows the migration of the compressional front eastward. The quaternary extension is led by NNW-SSE trending normal faults, which locally controls the development of intermountain basins (Bally et al., 1986; Calamita et al., 1994; Lavecchia et al., 1994; Chiaraluce et al., 2003; Buttinelli et al., 2021). The study area is basically characterized by Mesozoic pelagic successions (Umbria-Marche basin) and their Cenozoic terrigenous covers imbricated in several east verging thrust units, generally elongated in the N-S and NW-SE directions, with a very complex structural setting of the shallowest portion of the crust. The identification of mainshocks' causative faults and the role of the reactivation of pre-existing structures during the Umbria-Marche sequence is still debated in the literature (e.g., Barchi et al.,1998; Basili & Meghraoui 2001, Chiarabba & Amato, 2003, Chiaraluce et al., 2003; 2005). Calamita et al. (2000) interpreted the seismogenic low-angle normal fault as an inverted thrust ramp in the basement at a depth between 5 and 10 km; Chiaraluce et al.(2003) suggest that main shocks nucleated close to the intersection of normal faults and old thrusts. Boncio & Lavecchia (2000) suggests that normal faults cross-cut previous thrusts. Pizzi et al. (2002) suggest that the Colfiorito fault system is a neotectonic feature which, differently from other Apennines normal faults (Vettore, Norcia, Assergi), does not re-use previous Mesozoic or Cenozoic structures. Moreover, the evidence of primary faults reactivated at the surface, given the limited magnitude of the mainshocks, is still a matter of debate (Cello et al., 2000, Boncio & Lavecchia, 2000, Basili et al., 1998).