Control of geometry and kinematics on the state of stress of subduction zones: an application to the Mediterranean region

Knowing the stress field at subduction zones is fundamental as here is released most of the seismic energy in the Earth. In particular, most M W > 8.0 earthquakes originate at shallow depths along the frictional interface between subducting and overriding plates. This observation emphasizes the crucial role played by the geologic-time scale dynamics of convergent margins over the short-time scale seismogenic processes. Despite an obvious relevance to seismic hazard, knowing the driving forces generating the stress field at subduction zones is a long-standing problem. In this thesis, by means of 2D and 3D numerical viscoelastic models, I simulated the stress field in convergent plate margins to evaluate which properties control subduction dynamics. Models are built to evaluate the contribution of plate kinematics, geometry of the system, rheology and gravitational forces to the definition of the present-day stress field at different subduction zones. This has been achieved with the development of several sets of generic (i.e., not simulating specific subduction zones) 2D and 3D models. The aim is to analyze the interaction between the subducted slabs and the geodynamic forces (e.g., slab pull, mantle flow, plate convergence) that stress the system, to reproduce the observed stress fields measured in different subduction zones worldwide for both the upper and lower plates at crustal depths and for intermediate and deep subducted lithosphere. The interaction between subducting slabs and the viscosity jump at upper-lower mantle transition has been also investigated. Although generic, model geometries are consistent with natural geometries observed in real subduction zones worldwide. Modelling results are compared with stress data available in the world stress map database for different convergent margins. To define the stress field affecting the subducting plates, special attention must be paid to the choice of the righteous initial parameters, since from them depend the delicate balance between the applied tectonic forces and the geometric characteristics of the whole system. For this reason and to validate or reject the observations made for the general cases, the central Mediterranean subduction system was chosen to model a natural subduction zone. Building and constraining a model requires the knowledge of the real system. Subduction zones are primarily described by their geometry, and today the slab interfaces in the Mediterranean are still uncertain. I defined them reviewing and integrating literature data from various disciplines, collecting geometries into a specifically designed database. Unlike similar databases already available in the literature, in the database that I contributed to build the subduction interfaces are fully-parametrized, i.e., characterized by geometric (strike, dip, depth), kinematics and dynamic (rake, slip rate, seismic coupling, maximum earthquake magnitude) parameters. The database so designed, and its on-line publication makes it a valuable tool for the geometric description of active subductions in the Mediterranean area and provide the basis to investigate their seismic hazard.