Fault delineation and regional stress direction from the analysis of background microseismicity in the southern Apennines, Italy

The present active faults and stress field acting in the southern Apennines (Italy), a structurally complex area characterized by high seismic potential, are studied by analyzing the background microseismicity (M<=3). We used a microearthquake data set consisting of 1312 events that occurred from August 2005 to April 2011 by integrating the data recorded at 42 seismic stations of various networks. The refined seismicity locations and focal mechanisms delineate a system of northwest-southeast striking normal faults along the Apenninic chain and an approximately east-west oriented strike-slip fault transversely cutting the belt. The seismicity along the chain does not occur on a single fault but in a volume, delimited by the faults activated during the 1980 Irpinia M6.9 earthquake on subparallel predominantly normal faults. Results show that the recent low magnitude earthquakes belong to the background seismicity, and they are likely generated along the major fault segments activated during the most recent earthquakes, suggesting that they are still active today, 30 years after the mainshock occurrences. The stress inversion from the whole data set suggests that a unique anti-Apenninic extensional regional stress field could explain the two different faulting styles characterizing the earthquakes that occur along the chain and the east-west fault dissecting the belt. On the other hand, the results obtained by separately inverting the Irpinia and the Potenza clusters indicate a more complex model that would predict a change from a normal-faulting regime, acting in the inner sector of the chain, to a strike-slip regime moving eastward and down-depth in the Potenza area.