3d rheological modelling in the aegean region and its importance for the seismotectonics of the area

Resumen

The main topic of this Ph.D. research is the thermo-rheological modelling of the broader Aegean Region, for the purpose of seismotectonic and seismic hazard characterization. The concept behind this project resides in the link between rheological parameters obtained from the modelling and the properties of the corresponding rocks in terms of seismogenic behaviour. For the modelling, two dominant deformation mechanisms have been considered, namely the frictional sliding and the power-law creep representing, respectively, the brittle and the ductile behaviours. Literature data have been collected in order to define the proper range of values for most of the input parameters in the rheological constitutive equations. Additional geodynamic and geological considerations have been taken into account for determining the values of parameters such as the friction coefficient, the pore fluid pressure and the tectonic regime. Prior to the reconstruction of the strength envelopes, a detailed sensitivity analysis on the variability of the input parameters and their influence on the main thermo-rheological modelled properties has been carried out. The results of the sensitivity tests indicate that thermal-related parameters are the most influential ones for the BDT properties (depth and strength in particular). Dedicated and specific scripts have been developed in a Matlab environment for the purposes of the thermo-rheological modelling. In a first stage, 1D strength envelopes have been realized for specific test sites in the Aegean Region and have also been compared with the depth distribution of relocated seismicity, in order to test the precision and reliability of the correspondence between the BDT depth and the cutoff depth of seismicity. In a second phase, 2D rheological pseudo-sections have been reconstructed along several selected transects, belonging to different geodynamic settings. Particular care has been devoted to the comparison between continental and oceanic subduction settings. The results highlighted the occurrence of a deeper brittle layer below the shallowest BDT in the continental collision sectors, differently from the oceanic subduction setting. The third and last stage of the modelling consisted in the reconstruction of a complete 3D thermo-rheological model for the whole study area. Taking as a reference the subduction zone, it can be firstly observed that the BDT is much shallower (between 10 and 20 km, always in the upper plate) in the internal sectors with respect to the external areas, where the transition lies at ~40 km in the oceanic lithosphere of the lower plate, and at ~35 km in the continental sectors. Secondly, the strength and temperature at the BDT are generally well correlated with the BDT depth, meaning that strength values < 100 MPa characterize the internal sectors, while values up to 1 GPa are associated to the BDTs in the oceanic lithosphere. In terms of interpretation and explanation of the thermo-rheological results, all the models realized suggest that the main control on the BDT depth is exerted by the surface heat flow and the corresponding geothermal gradient. The results of the modelling have been successively applied mainly to the fields of seismotectonics and geodynamics for: i) estimating the maximum expected magnitudes of the major seismogenic sources in the Aegean Region; ii) calculating the seismic strain rates for selected volumes and iii) estimating the values of the total integrated strength for the tectonic plates in the study area. To conclude, this project has focused on the rheological characterization of the Aegean Region, being the first to propose a complete 3D model of the BDT depth and associated strength and temperature. This research has also demonstrated how the rheological properties can effectively be linked to the seismogenic processes, thus representing a valid tool for improving the seismotectonic characterization and, finally, the seismic hazard assessment.