The geometrical and mechanical features of alluvial deposits have a major influence on seismic-wave propagation and amplification. However, for alluvial basins located in densely urbanized areas, the surface structures such as buildings could influence seismic-wave propagation near the free surface. In this article, the influence of surface structures on seismic-wave propagation is analyzed numerically in the case of an actual two-dimensional (2D) shallow basin.

At a local scale, the vibration of a surface structure can induce a seismic wave field in the surficial soil layers. At the scale of an alluvial basin, the site-city models considered herein show that the city effect can lead to a significant seismic wave-field modification when compared to the free-field case. The coincidence between the fundamental frequencies of the soil layers and eigenfrequencies of the surface structures is a key parameter to investigate site-city interaction. When comparing simplified site-city models (Kham et al., 2006) to the basin-city model, the influence of the lateral heterogeneities on the site-city interaction is found to be significant. Indeed, the seismic wave field radiated by the city appears to be trapped within the alluvial basin, and specific directivity features are found for this wave field. The influence of site-city interaction on the free-field seismic hazard may then be significant. The effects of the site-city interaction are beneficial in some parts of the city or detrimental in other parts (especially city boundaries). These effects strongly depend on the urban configuration (city heterogeneity, building density, etc.). Finally, the full characterization of the seismic wave field in densely urbanized areas could often raise the need for investigating site-city interaction and consider such parameters as basin and city fundamental frequencies, building density and city arrangement, as well as basin effects combined with the seismic wave field radiated by the city.

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