In 1997, the Umbria-Marche Apennines (central Italy) was struck by a series of moderate magnitude normal faulting earthquakes, accompanied by a prolonged sequence of aftershocks. Seismological analysis revealed a ~40-km-long NW trending structure composed by several low-angle normal fault segments. In this paper we present the three-dimensional (3-D) Vp and Vp/Vs structure of the fault zone determined by the inversion of P and S wave arrival times from 1982 aftershocks. The high-resolution tomographic images reveal strong heterogeneity along and across the fault zone and allow us, along with the 3-D location of earthquakes, to define the fault geometry at depth and to improve our understanding on the seismotectonics of this part of the northern Apennines thrust-and-fold belt. The relocated aftershocks suggest that the main ruptures occurred on planes slightly steeper than previously hypothesized and at depths shallower than those evidenced by 1-D locations. We find that the preexisting structure, which developed during the Neogene compressional tectonics, plays an important role in the earthquake generation and rupture evolution of the normal faulting events. We propose that the main shocks, whose hypocenters are all located at ~6 km depth, originated along a shallow decollement of the Meso-Cenozoic cover, thrust over the metamorphic basement. Geometrical discontinuities and lateral steps along the inherited structures control the lateral fault extent and determine fault segmentation. This study offers a good example of how lithological heterogeneities and the complexity of the upper crust structure strongly affect the faulting process.