Spatial variability of the urban ground motion in a highly heterogeneous site-city configurations

Free-field conditions on the ground surface are widely used to assess the local seismic response. Nevertheless, this assumption is a very strong approximation in urban areas since the dynamic interaction between the urban agglomerate and the soil cannot be neglected. Several studies evidenced the transmission of vibrations from the buildings to the soil and the multiple interactions between a building network and the site through so-called Site- City Interaction (SCI). This paper focuses on the spatial variability of the seismic ground motion due to SCI in the case of highly heterogeneous site-city configurations. The Fosso di Vallerano valley in Rome (Italy) has been chosen as a case study because of the high heterogeneity of the deposits which characterize the local geological setting as well as the intensive urbanization. A proper 2D numerical modelling (Finite Elements Method) of the seismic response in free-field conditions as well as with the city agglomerates (SCI-based condition) has been performed. The numerical results show a strong spatial variability of the wave field induced by the presence of the buildings and their interaction with the heterogeneous soil. The SCI effect causes a strong or global reduction of ground motion, amplification levels and kinetic energy close to the buildings. Induced perturbations affect the ground motion along the surface of the model, increasing the amplification values and the kinetic energy in the vicinity of the buildings. These findings highlight that buildings generate significant variability effects all around their foundations as well as in the entire alluvial basin.