A note on central-northern Marche seismicity: New focal mechanisms for events recorded in years 2003-2009

A focal mechanism study was carried out for seismic events that occurred in the central-northern Marche region in years 2003 to 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 (INGV). 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.