GNGTS 2019 - Atti del 38° Convegno Nazionale

GNGTS 2019 S essione 1.1 7 edges to induce the slip. Instead, faults are assumed alternatively locked or unlocked in space and time (Doglioni et al. , 2011). When unlocked, fault’s edges move each other according to the applied far-field boundary conditions, loads, and internal stresses. A fully coupled, isotropic, linear poroelastic rheology describes the evolution of stresses and strains inside the medium (Wang, 2000), whose elastic and hydraulic parameters were derived from literature data and geophysical measurements available over the study areas. The mechanical boundary conditions consist of roller supports, orthogonal to the bottom and the sides of the model (Fig. 2) while the upper boundary is free to move in all directions. Applied forces consist of gravity load, and uniform shear traction applied at the model base (black arrows in Fig. 2) and directed north-eastward. The latter simulates the mantle convection in the Tyrrhenian asthenosphere and the Adriatic slab rollback, and it is intended to explain the present-day contraction-extension pair across the Apennine chain (Barba et al. , 2008; Carafa et al. , 2015). Simulations include an interseismic, coseismic and postseismic phase. In the interseismic phase, the lithostatic load and the interseismic tectonic load are simulated by activating the northeast-oriented basal shear tractions (black arrows in Fig. 2) (Finocchio et al. , 2016) to simulate the hypothetical crustal deformations and stresses gathered in the time interval between two earthquakes of similar magnitude and happening on the same fault segment. During this phase, the deeper fault segment (segment n°1 in Fig. 2) is assumed unlocked to simulate the effect of a fault segment steadily shearing during the interseismic phase, while the upper segment (segment n°2 in Fig. 2) is locked (Doglioni et al. , 2011; Scholz, 2019). Drained conditions are assumed for the fluid phase, and excess pore pressures are neglected. In the coseismic phase, the earthquake nucleation is simulated by instantaneously unlocking the upper part of the fault (segment n°2 Fig. 2), keeping both the mechanical boundary conditions and loads applied in the interseismic phase. Undrained conditions are assumed in this phase to allow for the development of excess pore pressures (hereinafter Δp) caused by the nearly-instantaneous fault dislocation. In the postseismic phase, the pore pressure excess caused by the earthquake dislocation dissipates over time because of pore fluid diffusion. The calculation time is two years. Boundary conditions and applied loads are kept constant, and the upper part of the fault (segment n°2 Fig. 2) is assumed unlocked. During this phase, the evolution of fluid pore pressures, stresses, and displacements caused by the pore fluid diffusion are monitored over time. Results. In the interseismic phase, the action of both gravitational and tectonic forces modulates the stress and strain field for normal- and reverse-fault events. The applied shear tractions at the model’s base (Fig. 2) cause the elongation of the left part of the model, with a reduction of both stresses and strains, and the compression of the right part of the model, with an increase of both stresses and strains. The assumed interseismic shearing of the deep fault segment at depth (segment n°1 in Fig. 2) modifies locally the displacement, strain and stress field. For the L’Aquila 2009 normal-fault event, the interseismic displacement vectors (white arrows in Fig. 3a) emphasise the shearing of the deep fault segment. Dilation occurs locally at depth in the hangingwall (Fig. 3b), i.e., at the contact between the unlocked and locked portions of the fault segment, while volumetric contraction develops up to two-kilometre depth, because of the interseismic ground subsidence caused by the fault shearing. For the Emilia 2012 reverse-fault event, the interseismic shearing produces displacements, volumetric strains and stresses opposite to those obtained for the normal-fault event. Displacements are upward oriented (Fig. 3c), causing the volumetric contraction of a wide area of the hangingwall (Fig. 3d), at the contact between the locked and unlocked portions of the fault segment. In the coseismic phase, the earthquake dislocation is simulated by unlocking the shallow part of the fault segment (segment n°2 in Fig. 2). For the L’Aquila 2009 normal-fault event, the coseismic deformation pattern (Fig. 3e) highlights the collapse of the hanging wall and the