Modelling the S-waves velocity spatial variability within the Amatrice Basin (central Italy) to capture the dominant propagation modes

The studied area is the Amatrice Basin, a NW-SE morphostructural intermountain depression of the central Apennine, formed during the Plio-Pleistocene extensional tectonic regime. Many hamlets located inside the basin suffered cumulative damages after the August 24th, 2016, Mw 6.0 earthquake and the following aftershocks. To complicate the seismo-tectonic setting, fluvial terraces, ridges, hillsides, morphological steps can be distinguished from a geomorphological viewpoint in the area. This complex framework was investigated with the aim of obtaining new insights from the role of layering in influencing the upward propagation of seismic waves, highlighting the possible occurrence of focusing, reflection, refraction and/or amplification effects in the basin. Thus, this scientific contribution aimed firstly at creating a refined mechanical model, able of integrating geological and geophysical (refraction, seismic tomography, surface waves methods measurements) data available for the uppermost hundreds of meters of the basin subsoil with the deep geological-geophysical prospecting data (such as: available seismic reflection-refraction profiles). Once data have been organized in a unique environment, where faults, stratigraphic boundaries and geophysical attributes have been modelled, a posteriori validation of the relationships between stratigraphies and S-waves velocities variability with depth and with space has been carried out by comparing the Rayleigh waves ellipticity computed for selected modelled seismo-stratigraphies with available noise measurements (HVSR curves). The adopted procedure also allowed investigating the role of geology in controlling the seismo-stratigraphic constraints in the Probabilistic Seismic Hazard Assessment (PSHA) perspective (i.e., analysing the influences of seismic bedrock depth assumptions and the S-waves velocity spatial variability). Thus, the proposed method constitutes a new approach in geological modelling devoted to these applications. The final product will be a 3D model useful for numerical simulations able to quantitatively estimating the seismic response in the basin, taking into account the particular geological and geophysical characteristics of the area.