Low seismicity hints at the accommodation zone between large seismogenic sources in the central-southern Apennines (Italy)

We analyze seismicity occurred in the 2007-2011 period on the border between the NW-SE striking central and southern Apennines (central Italy), characterized by the transition between two aligned antithetic normal fault systems. These regional fault sets include large seismogenic sources, causative of major earthquakes in the area, SW-dipping to the north, NE-dipping to the south, respectively. We (a) investigate the accommodation zone and the linkage between the SW-dipping Aremogna-Cinque Miglia (to the north) and NE-dipping Boiano Basin (to the south) sources, and (b) test whether the transfer zone model applies to the central-southern Apennines border. The epicentral distribution of the relocated earthquakes (1.6<MD< 3.4) well matches events occurred in the area in the previous ten years. Seismicity is made of single events and low magnitude swarms, with hypocenters within the upper 12-13 km. Swarms are located along a ~NNE-SSW trend, coincident with a section of the Ortona-Roccamonfina Line, a regional transverse lineament, and encircle the area where the accommodation zone would occur. Focal mechanisms of single events (MD>3.0) show both dip-slip and strike-slip motion, with T-axes NE-SW striking, consistent with the large-scale stress field controlling the Apennine Chain. Focal mechanisms of the swarms’ most energetic events show dip-slip motion, with T-axes ~NNW-SSE striking, coherent with local NW-SE extension hypothesized in this sector. Based on spatiotemporal characteristics of the seismicity, geometry and kinematics of active faulting in the region, and results from previous geophysical studies, we hypothesize (a) an accommodation zone between the Aremogna-Cinque Miglia and Boiano Basin sources, and (b) the activity of such linkage along the Ortona-Roccamonfina Line. We infer that the dip switch between the two antithetic seismogenic normal fault systems could also result from the rheologic and tectonic control exerted by the passage between two diverse paleogeographic domains composing the border between the central and southern Apennines