The September 27, 2012, ML 4.1, Benevento earthquake: A case of strike-slip faulting in Southern Apennines (Italy)

• We studied a small intra-Apennine seismic sequence (MLmax 4.1) in Southern Italy.
• Our analysis reveals an unknown seismogenic source nearby Benevento.
• A roughly E–W striking fault plane with dip 75° NNW is constrained.
• Seated at mid-crust, the source has a right-lateral strike-slip kinematic.
• Strong analogies with the 1990–91 Potenza and 2002 Molise earthquakes are evident.

On September 27, 2012 at 01:08 (UTC) a ML 4.1 earthquake started a seismic sequence approximately 10 km east of the city of Benevento, in Southern Apennines (Italy). During the following four days, about 40 events with ML ranging between 1.3 and 4.1 were detected in the same area, where the seismic hazard is one of the largest of the Italian Peninsula and where several historical and destructive events took place. In order to investigate the seismicity spatio-temporal pattern and to identify the seismogenic source geometry, a detailed analysis was performed integrating data recorded at three different seismic networks. The earthquakes were relocated using the double-difference technique and focal mechanism solutions were obtained by the moment tensor inversion. Also, to better understand the rupture process, seismic source parameters were estimated and apparent source time functions were inverted to retrieve the slip distribution for the largest magnitude event. Our results show the existence in the study area of roughly E–W striking fault plane with a right-lateral strike-slip kinematics, seated at mid-crustal depths (10–20 km), revealing a characteristic seismicity quite different from that typically associated to the outcropping NW–SE-striking active normal faults that are responsible of moderate to large earthquakes in the Southern Apennines axial sector.In this work, we address questions concerning i) the presence in the Benevento area of a mid-crust seismogenic strike-slip fault, previously unrecognized; ii) its link to the regional seismotectonic setting; and iii) the existence of a strike-slip tectonic regime that uniformly extends in the footwall of the Apennines thrust at relevant depth, not only in the Apulian foreland, as demonstrated to date, but also under the mountain chain axial zone.