Fault geometries from the space distribution of the 1990-1997 Sannio-Benevento earthquakes: inferences on the active deformation in Southern Apennines

In this study, we analyze the recent (1990-1997) seismicity that affected the northern sector (Sannio-Benevento area) of the Southern Apennines chain. We applied the Best Estimate Method (BEM), which collapses hypocentral clouds, to the events of low energy (M-d max = 4.1) seismic sequences in order to constrain the location and geometry of the seismogenetic structures. The results indicate that earthquakes aligned along three main structures: two sub-parallel structures striking NW-SE (19901992, Benevento sequence) and one structure striking NE-SW (1997, Sannio sequence). The southernmost NW-SE structure, which dips towards NE, overlies the fault that is likely to be responsible for a larger historical earthquake (I-o (max) = XI MCS, 1688 earthquake). The northernmost NW-SE striking structure dips towards SW. The NE-SW striking structure is sub-vertical and it is located at the northern tip of the fault segment supposed to be responsible for the 1688 earthquake. The spatio-temporal evolution of the 1990-1997 seismicity indicates a progressive migration from SE (Benevento) to NW (Sannio) associated to a deepening of hypocenters (i.e., from about 5 to 12 km). Hypocenters cluster at the interface between the major structural discontinuities (e.g., pre-existing thrust surfaces) or within higher rigidity layers (e.g., the Apulia carbonates). Available focal mechanisms from earthquakes occurred on the recognized NW-SE and NE-SW faults are consistent with dip-slip normal solutions. This evidences the occurrence of coexisting NW-SE and NE-SW extensions in Southern Apennines. (C) 2003 Elsevier Science BY. All rights reserved.