RESONANT RESPONSE OF A SOFT SEMICIRCULAR CYLINDRICAL BASIN TO AN SH SEISMIC-WAVE

The resonant nature of surface motion above a soft semi-circular cylin drical basin subjected to SH plane waves is demonstrated theoretically from the rigorous formulation and solution of the problem. The connec tion of the resonances of this ''open structure'' with a nonrigid bott om is established with those of the ''closed structure'' having a rigi d bottom. The resonances are shown to be the manifestation of the exci tation of the normal modes of the bedrock-basin system. The form of th ese modes is established. Exact as well as approximate pro- cedures ar e given for computing the characteristic frequencies Omega. The rigoro us approach shows that the Omega are generally complex. The rigorously computed Omega are used to validate the 1D, Bard and Bouchon, Rial, a nd new approximate procedures for computing the (real part of the) Ome ga. Dropping the rigid bottom assumption enables a computation to be m ade of the response at resonance, which includes all basin-bedrock int eractions. The latter can be dangerously underestimated if the resonan ces are assumed to be located where the rigid bottom assumption predic ts them to be. The main features of the ground motion are explained by the modal analysis, notably those concerning the spatial variability along the ground in different frequency ranges and its dependence on t he driving field. Many of these features apply to basins of more gener al shape.