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The backbone of the Southern Apennines is perhaps the largest seismic moment release area in Italy. The region is dominated by an extensional regime dating back to the Middle Pleistocene, with maximum extension striking SW-NE (i.e. orthogonal to the mountain belt). The full length (~ 200 km) of the mountain range has been the locus of several destructive earthquakes occurring in the uppermost 10-12 km of the crust. This seismicity is due to a well documented normal faulting mechanism. Instrumental earthquakes (e.g. 5 May 1990, 31 Oct 2002, 1 Nov 2002; all M 5.8) that have occurred in the foreland, east of the Southern Apennines, have posed new questions concerning seismogenic processes in southern Italy. Although of moderate magnitude, these events unveiled the presence of E-W striking, deeper (13-25 km) strike-slip faults. Recent studies suggest that these less known faults belong to inherited shear zones with a multi-phase tectonic history, the most recent phase being a right-lateral reactivation. The direction of the maximum horizontal extension of these faults (in a transcurrent regime) coincides with the maximum horizontal extension in the core of the Southern Apennines (in an extensional regime) and both are compatible with the general framework provided by the Africa-Europe convergence. However, the regional extent along strike of the E-W shear zones poses the issue of their continuity from the foreland towards the thrust-belt. The 1456 (M 6.9) and 1930 (M 6.7) earthquakes, that occurred just east of the main extensional axis, were caused by faults having a strike intermediate between the E-W, deeper strike-slip faults in the foreland and the NW-SE-trending, shallower normal faults in the extensional belt. Hence, the location and geometry of these seismogenic sources suggests that there could be a transition zone between the crustal volumes affected by the extensional and transcurrent regimes. To image such transition, we built a 3D model that incorporates data available from surface and subsurface geology (published and unpublished), seismogenic faults, seismicity, focal mechanisms, and gravity anomalies. We explored the mechanisms of fault interaction in the Southern Apennines between the extensional upper portion and the transcurrent deeper portion of the seismogenic layer. In particular, we studied (a) how the reactivation of regional shear zones interacts with an adjacent, although structurally independent, extensional belt; (b) at what depth range the interaction occurs; and (c1) whether oblique slip in earthquakes like the 1930 event is merely due to the geometry of the causative fault, or (c2) such geometry and kinematics are the result of oblique slip due to fault interaction. We propose that (a) the 1456 and 1930 earthquakes are the expression of the transition between the two tectonic regimes, and that (b) these events can be seen as templates of the seismogenic oblique-slip faulting that occurs at intermediate depths between the shallower extensional faults and the deeper strike-slip faults. These findings suggest that a transtensional faulting mechanism governs the release of major earthquakes in the transition zone between extensional and transcurrent domains., The 3D model whose rendering is shown in this poster was performed by inputting data from the seismogenic sources into 3D GeoModeller (© BRGM and Intrepid Geophysics, 2004). Assistance from Intrepid Geophysics during the construction of the 3D model is highly appreciated. Work financially supported by project DPC-INGV S2 (U.R. 1.1 and 2.4).

Most of Southern Italy is underlain by a foreland consisting of African Palaeozoic basement, in turn overlain by deformed Meso-Cenozoic terranes. This crustal block, known in the literature as Adria or African Promontory, dips towards SW and is strongly involved into the build-up of the Central and Southern Apennines. From existing seismic reflection data, oil exploration wells, gravity and magnetic data, it is possible to reconstruct the geometry of the Apulian carbonate platform (known as Apula), a paleogeographic domain within the Tethyan Ocean, representing the original sedimentary cover of the Palaeozoic basement. Within Apula, several major surfaces were active during mountain building. Despite the depths involved, the lateral facies variations and the complex structure of the Southern Apennines nappe, such surfaces were re-activated thanks to the thickness of the Apulian platform, which represents an internally coherent, single tectonic element. In places, the seismic data well show the overall top of the Apulian platform, displaying a multiple faulting pattern. An earlier, mainly extensional fault system was generated before and during the sinking of Adria underneath the Eurasian lithosphere. A later system reasonably shows the structural expression of the main phase of the Apennines’ mountain building. The region also includes a major seismic record, with several destructive earthquakes (both historical and instrumental) and long-term geomorphic indicators. Such evidences hint at the ongoing processes that contribute to the mountain building. This is enforced primarily by means of large normal fault systems, having various strike and importance, that generate the long- and short-wave landscape as a result of active deformation. Here we discuss the observed interplay between the overall tectonic fabric shown at the top of the Apulian platform, the structural arrangement of seismogenic sources that are well constrained by geological and seismological data and the remarkable E-W regional shear zones cross-cutting the Apulian remnants E of the Apennines leading edge. Most of these tectonic elements are inherited by the Apulian platform and, in places, cut through the overburden, interfering with younger structures. Additionally, they are inferred from the earthquake record in areas where a causative source is not unequivocally identified yet a plausible active fault within Apula or the Paleozoic basement falls within the seismogenic layer.

We show and discuss the similarities among the 2016 Amatrice (Mw 6.0), 1997 Colfiorito-Sellano (Mw 6.0-5.6) and 2009 L’Aquila (Mw 6.3) earthquakes. They all occurred along the crest of the central Apennines and were caused by shallow dipping faults between 3 and 10 km depth, as shown by their characteristic InSAR signature. We contend that these earthquakes delineate a seismogenic style that is characteristic of this portion of the central Apennines, where the upward propagation of seismogenic faults is hindered by the presence of pre-existing regional thrusts. This leads to an effective decoupling between the deeper seismogenic portion of the upper crust and its uppermost 3 km.The decoupling implies that active faults mapped at the surface do not connect with the seismogenic sources, and that their evolution may be controlled by passive readjustments to coseismic strains or even by purely gravitational motions. Seismic hazard analyses and estimates based on such faults should hence be considered with great caution as they may be all but representative of the true seismogenic potential.