A geological model for the Colfiorito earthquakes (September-October 1997, central Italy)

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(degrees)) toward SW, which appears to be independent from the presence of pre-existing thrust planes.