Seismic hazard in urban environments : Can man modify the hazard ?

It is most generally admitted that seimic ground motion is the convolution of source, path and site effects. The question addressed here is whether a fourth term, called "Site-City Interaction" (SCI), should be added for densely urbanized areas, corresponding to feedback effects from the shaken buildings in the soil. After a short overview of various, consistent observations, several specific experimental and numerical results are briefly summarized, that clearly evidence the wave radiation from isolated buildings and the possibilty of interactions between close buildings as well. The seismic response of a simple "city model" represented by a group of buildings is then addressed through a series of numerical simulations, which allow to determine under which conditions such SCI effects may be significant. These findings are consistent with a very simple "rule of thumb" approach based on the comparison between kinematic energies in soils and buildings, pointing to the significance of SCI effects for dense cities where building and soil periods coincide. The last section thus discusses the practical consequences of such SCI effects, and some possible directions to get unambiguous experimental evidence in real cities.