Seismicity at the border between the Southern and Central Apennines (Italy): a review

We review the instrumental seismicity at the boundary between the Southern and Central Apennines with the aim of detecting the active structures and clarifying the seismotectonic framework of this area of the chain. Single events with M<3.0, low magnitude (M<4.0) seismic sequences (1997-1998 and 2001), and swarms (1999, 2000 and 2005) characterize the seismicity of this area. Earthquakes occur within the upper 15 km of the crust, prevalently aligning along the Apennine Chain in a NW-SE axis. Strain analyses indicate that these events are related to the main NE-SW extension processes that affect the Apennine Chain. They are concentrated in the following areas: (a) south of the seismogenetic source responsible for the 1915 destructive earthquake (2000 swarm), (b) between the faults of the 1984 and 1805 destructive events (2001 sequence), and (c) between the faults of the 1805 and 1688 events (1997-1998 seismic sequence). The 1999, 2000 and 2005 swarms occurred along the so-called Ortona-Roccamonfina alignment, which is the NNE-SSW belt separating the Central Apennines from the Southern ones. The distribution of these swarms and the focal mechanisms indicate the presence of active NE -SW faults related to a local NW-SE extension. This direction of extension was also observed in the 1997-1998 and 2001 sequences. The location of the NE -SW striking faults, responsible for the seismic swarms, suggest that some segments of the Ortona-Roccamonfina line may be a rupture response to both the NE -SW regional extension of the Southern Apennines, and to a NW-SE striking longitudinal extension. Our data indicate that the seismicity of the Apennines at the boundary between the central and southern sectors is related to both the eastward migration of the chain (the NE-SW extension) and to its progressive curvature and thinning (the NW-SE extension). The NE-SW extension is responsible for the major earthquakes whereas the lower energy seismicity is related to the NW-SE extension.