In this paper, we report on the velocity structure and seismicity of the northern-central Apennines. Recent seismological studies elucidate the crust and uppermost mantle structure in this peculiar and controversial portion of the central Mediterranean region. Quaternary extension develops on a main NW-trending seismic belt located along the Apennines range, at the boundary between the Adria and Tyrrhenian lithospheres, i.e. a complex setting in a more general framework of the Africa-Eurasia collision. This boundary is well defined at Moho depth by tomographic P-wave velocity models and receiver function analyses. The characteristics of the mantle vary between the Tyrrhenian and Adria domains, suggesting that the Tyrrhenian wedge is widely permeated by fluids due to the presence of subducted lithosphere at depth.

The seismic belt is fragmented into adjacent and laterally offset segments, whose length is in the order of tens of kilometres. This feature is consistent with the occurrence of moderate and large (M>6.0) normal faulting earthquakes. We hypothesize that the fragmentation derives from the extreme complexity of the pre-existing structure, where subsequent tectonic inversions, from the Mesozoic Tethys extension to the Neogene compression and then to the presently active extension, created a puzzling scenario of faults in the crust. Deep fluids formerly released by the subduction process and broadly stocked in the mantle wedge up-raise in the crust favouring the re-activation of the pre-existing structures in the extending Apennines range.