Dottorato di Ricerca in Scienze della Terra, Università “Carlo Bo”, Urbino, Italy

A focal mechanism study was carried out for seismic events that occurred in the central-northern Marche region between 2003 and 2009. The study is based on earthquake data, having magnitude M ≤ 3.5, recorded by the seismic network of the Istituto Nazionale di Geofisica e Vulcanologia. The results show that events tend to cluster into different sectors, each being characterized by a roughly homogeneous seismological behaviour. The new data confirm the occurrence of an extensional stress regime in the axial zone of the mountain belt, generally characterized by vertical or sub-vertical P-axes. On the other hand, a different stress regime appears to dominate in the outer Apennine sector, extending from the high valley of the Marecchia River to the foothills west of Ancona. Here, the well-known extensional domain of the axial zone of the Apennine mountain belt gives way, through an area characterized by oblique-slip faulting and variably plunging P-axes, to a regime dominated by roughly horizontal P-axes along the Adriatic onshore. Roughly horizontal P-axes characterize also the adjacent Adriatic Sea sector, although some variability in the maximum compression direction occurs in the offshore area.