Active faulting and Deep-seated Gravitational Slope Deformation (DGSD) constitute common geological hazards in mountain belts worldwide. In the Italian central Apennines, km-thick carbonate sedimentary sequences are cut by major active normal faults which shape the landscape generating intermontane basins. Geomorphological observations suggest that the DGSDs are commonly located in the fault footwalls. We selected five mountain slopes affected by DGSD and exposing the footwall of active seismic normal faults exhumed from 2 to 0.5 km depth. We combined field structural analysis of the slopes with microstructural investigation of the slipping zones from the slip surfaces of both DGSDs and major faults. The collected data show that DGSDs exploit pre-existing surfaces formed both at depth and near the ground surface by tectonic faulting and, locally, by gravitational collapse. At the microscale, the widespread compaction of micro-grains (e.g., clasts indentation) forming the cataclastic matrix of both normal faults and DGSDs is consistent with clast fragmentation, fluid-infiltration and congruent pressure-solution mechanisms active at low ambient temperatures and lithostatic pressures. These processes are more developed in the slipping zones of normal faults because of the larger displacement accommodated. We conclude that in carbonate rocks of the central Apennines, DGSDs commonly exploit pre-existing tectonic faults/fractures and, in addition, localize slip along newly formed fractures that accommodate deformation mechanisms similar to those associated to tectonic faulting. Furthermore, the exposure of sharp slip surfaces along mountain slopes in the central Apennines can result from both surface seismic rupturing and DGSD or by a combination of them.

According to the concept of the seismic cycle, earthquakes result from the strain accumulation over a variable decade to millennial period, i.e., the interseismic stage, followed by a sudden stress release, i.e., the coseismic stage, eventually evolving in the postseismic stage. Common analytical and numerical approaches simulate interseismic, coseismic and postseismic stages independently. Often, coseismic models constrain the slip of single or multiple planar sources to fit the available geodetic and InSAR measurements to reproduce fault geometry, slip and regional deformation, regardless the origin of the interseismic forces. We developed a numerical model linking the ongoing interseismic viscous deformation at depth with the coseismic brittle episodic behavior of the upper crust. Our model assumes a brittle upper crust where the fault is locked, and a ductile lower crust, where the fault is steadily shearing. This approach is developed to model typical extensional and compressional earthquakes in Italy including the forces acting during the interseismic period, i.e., the lithostatic load and the horizontal stress field. We adjusted the setup of our model to simulate the interseismic, coseismic and postseismic phases of three seismic events in Italy, two extensional (2009 L’Aquila Mw 6.1 and 2016 Amatrice-Norcia Mw 6.5) and one contractional (2012 Emilia Mw 6). The results of our analysis, compared with the available geodetic and InSAR data, show that the proposed numerical model can reproduce the seismic cycle associated with the investigated events. The modeling provides evidence of interseismic dilatancy above the brittle-ductile transition at the bottom of the locked fault plane in the extensional tectonic setting; coseismic fault motion is triggered by the hangingwall gravitational collapse that recovers most of the interseismic dilatancy formed almost orthogonal to the fault. Vice versa, in contractional tectonic settings, the interseismic horizontal stress accumulates elastic energy in the crustal volume above the bottom of the locked fault; coseismic deformation recovers the elastic energy stored in the hangingwall. The two different settings generate a deformation in favor of gravity in extensional tectonic environments and against gravity in contractional tectonic environments.