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

In this study, surface and subsurface geological data are integrated with s eismological data in order to reconstruct a structural model for the Septem ber-October 1997 Colfiorito earthquakes. The seismic sequence is mainly con trolled by two major SW-dipping normal faults outcropping in the area (M. P ennino-M. Prefoglio and M.Civitella-Preci faults). The activated faults det ach, at depth, on a common east-dipping low-angle normal fault, the Altotib erina Fault (AF). The AF is interpreted as the base of an active hangingwal l 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 relat ed to the eastward-deepening geometry of the AF detachment. The seismic fau lt planes, inferred from focal mechanisms and aftershock distributions, are characterised by a moderate dip (average 40(degrees)) toward SW, which app ears to be independent from the presence of pre-existing thrust planes.