  For the analysis of seismic wave amplification, modal methods are interesting
tools to study the modal properties of geological structures. Modal approaches
mainly lead to information on such parameters as fundamental frequencies and
eigenmodes of alluvial basins. For a specific alluvial deposit in Nice
(France), a simplified modal approach involving the Rayleigh method is
considered. This approach assumes a set of admissible shape functions for the
eigenmodes and allows a fast estimation of the fundamental frequency of the
basin. The agreement between modal numerical results and experimental ones is
satisfactory. The simplified modal method then appears as an efficient mean for
the global vibratory characterization of geological structures towards
resonance.
