Structural setting of the 1688 Sannio earthquake epicentral area (Southern Italy) from surface and subsurface data

Abstract

Abstract We analysed the 1688 Sannio earthquake, one of the most destructive events that occurred along the axis of the Southern Apennines (Calore River valley, Italy). It was characterised by a very large damage area and by several ground effects. Nonetheless, there is still a lack of specific and multidisciplinary geological studies focused on the active tectonics of the area where this earthquake occurred. Therefore, this work is aimed at integrating subsurface and surface data to provide a new reconstruction of the present day structural setting and active tectonics of the region struck by the 1688 Sannio earthquake. We interpreted deep well logs and reflection seismic lines, and integrated them with the results of original geomorphic and mesostructural analyses and with new radiometric dating (40Ar/39Ar) of Pleistocene pyroclastic layers. The latest Pleistocene brittle deformation observed in the Calore valley suggests a NE dipping main fault related to a NW-SE oriented active extensional system (Calore River fault system: CRFS). This extensional system is tentatively interpreted as the seismogenic fault of the 1688 Sannio earthquake. The reconstruction of the deep structural setting of the study area, especially in correspondence with the CRFS, allowed the buried Apulia units to be identified. The active extensional fault system develops within a set of thrusts that strongly uplift the Apulia platform succession and possibly the underlying Paleozoic basement. At surface, instead, the extensional fault system projects within the most external parts of the Apennine unit, in proximity to its leading edge. A comparison with other extensional seismogenic sources of the Southern Apennines suggests that the occurrence of the described features could represent a key for the location of the major seismicity of the region and could provide an interpretative model for the identification of areas of possible seismic gap in Southern Italy.