Seismic activity in the transition zone between Southern and Central Apennines (Italy): Evidences of longitudinal extension inside the Ortona–Roccamonfina tectonic line

We analyze the 1997–2006 seismicity of the transition zone between Southern and Central Apennines, which
is one of the most active seismic areas of Italy. Our aim is to add information on the seismotectonic picture of
this area. Seismic activity is characterized by single events with Mb3.0 and low magnitude (Mb4.0) seismic
sequences (1997–98 and 2005) and swarms (1999, 2000 and 2001). Hypocenters are within the upper 15 km
of the crust. The epicentral distribution of the relocated seismicity shows that single events prevalently align
NW–SE along the Apennine chain axis. This seismicity is related to the main, NE–SW extension affecting the
chain. Single events concentrate also: at the south of the seismogenetic source responsible for the 1915
earthquake, where the 2000 swarm occurred; between the faults of the 1984 and 1805 events, where the
2001 sequence developed; between the faults of the 1805 and 1688 events, where the 1997–1998 seismic
sequence concentrated. The seismic swarms occurred in 1999, 2000 and 2005 are located inside the Ortona–
Roccamonfina structural line, which strikes NNE–SSW and separates the Central Apennines from the
Southern ones. The epicentral distribution of these swarms and focal mechanisms suggest the presence of
active NE–SW faults moving in response to a NW–SE extension. The results of the strain analysis on 52 wellconstrained
focal mechanisms evidence a prevailing NE–SW extension, corresponding to the large scale
stress field acting in the Apennine Chain, and a second-order NW–SE extension. This last direction of
extension was already observed in the 1997–98 and 2001 seismic sequences. The location of the NE–SW
striking faults responsible for the seismic swarms suggest that some segments of the Ortona–Roccamonfina
line are still active and move in response to both the NE–SW regional extension of Southern Apennines, and
to a NW–SE striking longitudinal extension.