Surface wave quantification in a highly heterogeneous alluvial basin. Case study of the Fosso di Vallerano valley, Rome, Italy

The Vallerano district in Rome is built on an alluvial valley with a very complex geological setting (highly heterogeneous soil deposits) due to the depositional activity of Tiber River tributaries ongoing since the Pleistocene age. A high-resolution engineering-geological model of the valley was reconstructed through field investigations, boreholes log-stratigraphies and geophysical investigations. 2D FEM numerical modelling of the alluvial valley allowed to assess the local response in terms of seismic wave propagation on surface, amplification function and cumulative kinetic energy. Soil layering and basin effects combine to induce a complex local seismic response. Different typologies of seismic waves are present in the seismograms and surface waves have been also detected. Retrograde Rayleigh surface waves have been identified and extracted through a procedure based on the time-frequency Normalized Inner Product. The spatial analysis of the extracted Rayleigh waves highlights that the local seismic response of the valley is strongly influenced by edge-generated surface waves. The results suggest that amplification is strongly related to the thickness of the soil deposits, whereas the elongated duration is more related to a layered basin structure. The soil layering and the basin effects on the generation of surface waves have been assessed and quantified.