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 M < 3.0 and low magnitude (M < 4.0) 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 well-constrained 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.