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 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.