Active tectonics and earthquake sources in the epicentral area of the 1857 Basilicata earthquake (southern Italy)

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The 1857 Basilicata earthquake (Imax=XI° MCS; Me=6.9) is one of the most destructive events that occurred in peninsular Italy; shaking effects and ground breaks were recorded over a large area extending from the Vallo di Diano (Campania) to the Val d'Agri (Basilicata) for a length of about 60 km and with a width of more than 10 km. Within this seismogenic belt, only another strong earthquake, with maximum intensities in the range of X° MCS (Me=6.4), occurred in 1561. In the epicentral area of the 1857 earthquake, two regional fault systems (i.e. the Val d'Agri and the Vallo di Diano fault systems) offset the main features of the southern Apennines fold and thrust belt; both systems show evidence of activity during Pleistocene times. The Vallo di Diano Fault System (DIFS) includes mostly NW–SE and WNW–ESE trending faults displaying long-term displacements of a few hundred meters; slip data from the latter faults record a kinematic transition from almost pure normal motion to dextral/oblique motion, whereas the NW–SE oriented faults are mostly dominated by normal/transtensional (sinistral) motion. The Val d'Agri Fault System (VAFS) is characterized by fault zones of different size; it is a kinematically coherent system including roughly N120° trending left-lateral strike-slip faults and N090°–N100° trending left-lateral transtensional faults. Inversion of fault slip data indicates that the stress field conditions responsible for the genesis and evolution of the two fault systems are quite different, with σ1 being: (1) sub-horizontal and WSW–ENE trending, in the case of the VAFS, and (2) sub-vertical, in the case of the DIFS. However, the two fault systems are characterized by a roughly N–S oriented extension, and by R-values indicating that σ1≌ σ2>> σ3. This suggests the possibility that, in these areas, permutations between the principal maximum and intermediate axes of the stress ellipsoid may have frequently occurred during the faulting process. In this paper, we present new data for both the VAFS and DIFS and discuss the inferred modes of interaction between the two fault systems; this, in turn, suggests possible implications for seismic hazard analyses (SHA) in this sector of the southern Apennines.