Journal of Seismology 4: 345–356, 2000. © 2000 Kluwer Academic Publishers. Printed in the Netherlands. 345 A geological model for the Colfiorito earthquakes (September-October 1997, central Italy) Paolo Boncio & Giusy Lavecchia Dipartimento di Scienze della Terra, Universit` a G. D’Annunzio, Chieti Scalo, Italy; e-mail: pboncio@unich.it, glavecchia@unich.it Received 9 March 1999; accepted in revised form 17 March 2000 Key words: earthquake, normal fault, seismic sequence, seismotectonics, Umbria-Marche Apennines Abstract In this study, surface and subsurface geological data are integrated with seismological data in order to reconstruct a structural model for the September-October 1997 Colfiorito earthquakes. The seismic sequence is mainly controlled by two major SW-dipping normal faults outcropping in the area (M. Pennino-M. Prefoglio and M.Civitella-Preci faults). The activated faults detach, at depth, on a common east-dipping low-angle normal fault, the Altotiberina Fault (AF). The AF is interpreted as the base of an active hangingwall block which is stretching toward NE. The decrease in maximum depth of the earthquake foci from the Colfiorito area (about 8 km) to the Sellano area (about 6 km), suggested by the available seismological data, could be related to the eastward-deepening geometry of the AF detachment. The seismic fault planes, inferred from focal mechanisms and aftershock distributions, are characterised by a moderate dip (average 40 ◦ ) toward SW, which appears to be independent from the presence of pre-existing thrust planes. Introduction The September-October 1997 Colfiorito earthquakes (central Italy) have been widely investigated from both seismological and geological point of views (Amato et al., 1998; Anzidei et al., 1999; Boncio and Lavec- chia, 1999; Calamita et al., 1999; Cattaneo et al., this volume; Cello et al., 1998; Cinti et al., 1998; Ek- strom et al., 1998; Galli et al., 1998; Stramondo et al., 1999). In this paper, through the integrated analysis of geological, geophysical and seismological data, we attempt a structural interpretation of the seismic se- quence, in the frame of a previously proposed (Boncio and Lavecchia, 2000) regional seismogenetic model. Seismotectonic framework Stress field and structural style of the Plio-Quaternary extensional tectonics The Umbria-Marche Apennines (U-M Ap.) of cent- ral Italy are affected by Plio-Quaternary extensional deformation, with mainly SW-dipping normal and normal-oblique faults and associated intramontane basins (Figure 1). The related stress field is tensional with SW-NE trending maximum tension (Calamita and Pizzi, 1994; Lavecchia et al., 1994). An east-dipping low-angle reflector has been re- cently pointed out in the northern Umbria region on the basis of NVR seismic data (CROP 03 project, Bar- chi et al. 1998). Barchi et al. (1998) and Boncio et al. (1998) interpreted such reflector as an eastward deepening active low-angle normal fault zone named Altotiberina Fault (AF) (Figure 1). The surface ex- pression of the AF zone consists of low-angle (average 30 ◦ ) synthetic splays bordering westward the Eastern Tiber basin (Brozzetti, 1995). The deep geometry of the AF has been detailed through the analysis of deep crust (CROP 03) and commercial seismic reflection profiles (Figure 2 and references therein). The geolo- gical interpretation of these profiles shows that the AF deepens eastward to a depth of 12–14 km; it has a stair- case trajectory with an average dip of about 30 ◦ , and represents the basal detachment of the intra-Apennine