A unified numerical model for the simulation of the seismic cycle in dip-slip environments: examples from Italy

According to the concept of the seismic cycle, earthquakes are the result of the strain accumulation in the earth's crust over a variable decade to millennial period, i.e., the interseismic stage (eventually evolving in a preseismic stage), followed by a sudden stress release at a crustal discontinuity, i.e., the coseismic stage, finally evolving in a postseismic stage (Scholz, 2019). Commonly, the seismic cycle is modelled with analytical and numerical approaches. Quasi-static analytical methods simulate the interseismic coupling, the coseismic dislocation and the postseismic relaxation independently and assuming an elastic, viscoelastic or poroelastic half-space. Often, these models impose the slip on single or multiple planar sources to infer fault geometry, slip distribution and regional deformations in order to fit the available geodetic or seismological measurements, often regardless of the magnitude and orientation of the interseismic gravitational and tectonic forces (Anderlini et al., 2016; Atzori et al., 2009). Numerical approaches allow simulating complex geometries in heterogeneous media and at different modelling scales, assuming elastic, viscoelastic, or elasto-viscoplastic constitutive laws. At small scale (i.e., at the scale of the single fault), numerical models often impose the slip on the fault plane to simulate independently the observed coseismic dislocation or the propagation of the seismic waves (Trasatti et al., 2011), or they adopt ad-hoc boundary conditions to investigate the interseismic stress accumulation or the postseismic relaxation for specific cases (Carminati and Vadacca, 2010). At medium-to-large scale, numerical models can jointly simulate the tectonic and seismic behaviour of mountain belts and subduction zones (Dal Zilio et al., 2018), but the resolution is not enough for a detailed simulation of a single earthquake event. Since fault activation is crucial for the understanding of earthquakes and their prediction, we contribute to the understanding of the seismic cycle associated to a single fault segment by developing a small-scale numerical model to simulate the long-term crustal interseismic deformation, the coseismic brittle episodic dislocation, and the postseismic relaxation of the upper crust within a unified environment for both normal and reverse fault events. Our model assumes a brittle upper crust where the fault is locked, and a ductile lower crust, where the fault is steadily shearing (Doglioni et al., 2011). This model is developed to simulate typical extensional and compressional earthquakes in Italy (Fig. 1a) and includes the forces acting during the interseismic period, i.e., the lithostatic load and the horizontal stress field (Finocchio et al., 2016). We adjusted the setup of our model to simulate the interseismic, coseismic and postseismic phases for two major seismic events in Italy, the 2009, Mw 6.1 L’Aquila normal fault earthquake (Fig.1b) and the 2012, Mw 5.9 Emilia-Romagna reverse fault earthquake (Fig. 1c). The results of our analysis, compared with geodetic and InSAR data from the literature, show that the proposed numerical model is able to reproduce the seismic cycle associated with the investigated events.