Based on both theoretical and numerical studies, this paper aims at quantifying the modification of the seismic ground motion in dense urban areas also referred to as the city-site effect. From the theoretical point of view, the scattering cross-section of a vibrating building is first calculated from classical soil-structure interaction (SSI) framework. Using the image technique, the mean free path l is approximated and compared to the wavelength in order to determine the wave propagation regime: single or multiple scattering. A numerical method is then used to investigate the seismic wave propagation in a random city in the strong multiple scattering regime. The city site effect is shown to play a significant role both on the mean response and on the fluctuations. Local resonance and very short correlation length of the scattered field with respect to the wavelength indicate a localization like regime. At last, engineering implications are drawn from these results.
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