Belt-Parallel Shortening in the Northern Apennines and Seismotectonic Implications.

Major seismic activity in the Northern Apennines concentrates in few zones, distributed in a pecu- liar way. It is argued that such context may be plausibly explained as an effect of belt-parallel shortening, which has caused oroclinal bending of the longitudinal ridges formed during the Late Miocene to Lower Pliocene evolutionary phase. The main effects of this process, developed since the upper Pliocene, have mainly affected the outer sectors of the belt. The major seismic sources have generated in the zones where different oroclinal bendings of adjacent ridges have produced extensional/transtensional deformation. In the inner side of the Northern Apennines, belt parallel shortening has occurred at a lower rate. The main effects have resulted from the shortening of the Albano-Chianti-Rapolano-Cetona ridge. In particular, the proposed tectonic setting may account for the moderate seismic activity that occurs in the Firenze, Elsa, Pesa, Siena and Radicofani ba- sins.