A soft linkage between major seismogenic fault systems in the central-southern Apennines (Italy): evidence from low-magnitude seismicity

We investigate the transfer zone and linkage between divergent extensional seismogenic fault systems on the border amid the central and southern Apennines (central Italy). These regional NW-SE striking sets include large seismogenic sources that caused major historical earthquakes (Mw≤7). The faults in the northern part of the study area dip to the southwest; those in the southern part dip to the northeast. The SW-dipping system (Abruzzi Apennines) terminates with the Aremogna-Cinque Miglia source; the NE-dipping system (southern Apennines) terminates with the Boiano Basin source.
To test whether the transfer zone model applies to the central-southern Apennines border, we analyzed and relocated seismicity that occurred from 2007 to 2011 between the Aremogna-Cinque Miglia and Boiano Basin sources, where we expect the transfer zone. Seismicity is made of independent events (Md<3.5) and low-magnitude swarms. West of the Apennines, hypocenters are located within the uppermost 12-13 km. Events and swarms that occurred east of the axis affect the 13-25 km below. West of the chain, focal mechanisms show T-axes striking ~NNW-SSE. East of the chain, T-axes strike ~NE-SW. This trend is consistent with GPS data. The hypocentral distribution of swarms located between the Aremogna-Cinque Miglia and Boiano Basin sources shows a ~NNE-SSW trend, coincident with part of the Ortona-Roccamonfina Line, a regional transverse lineament.
The spatial distribution of seismicity, the geometry and kinematics of active faulting in the region, and results from previous geophysical studies, allow us to contend the existence of a transfer zone between these seismogenic normal fault systems. Our data also allow us to recognize the activity of such transfer along the central part of the Ortona-Roccamonfina Line. We infer that reverse in dip polarity between the two normal fault systems could also result from the passage between the diverse tectonic units composing the border between central and southern Apennines.