L'Interaction site-ville: Un nouvel aspect de l'aléa sismique

The interaction between ground motion and structural motion affects a structure's dynamic behavior, especially if its foundation has been laid in soft soil. When a structure is submitted to external stress, a portion of its vibration energy gets restored at the level of the ground-foundation interface. This phenomenon is manifested by the creation of seismic waves propagating within the surface formations, in particular when the impedance contrast with the underlying substratum is significant and when vibration frequencies of the ground and structure are close to one another (the resonance phenomenon). It then becomes possible to question not only the consequences of this phenomenon on the scale of a city composed of multiple structures, but also the effect of continued urbanization on the breakdown of seismic risk. The objective of the present article is to provide a series of experimental and numerical arguments proving the relevance of studies conducted on this "site-city interaction". Following a summary of several previous observations, the case of an isolated structure is first treated by means of a numerical model calibrated on an original field experiment carried out in Greece. Once validated, the model is then applied to groups of buildings that simulate the actual urban layout of a Mexico City district. This same district also serves as the basis for a parametric study to identify the primary parameters controlling both the presence and magnitude of the site-city interaction. Moreover, a simple approach based on energy considerations is presented and successfully applied to the Mexico City district example.