Quantitative estimation of seismic site effects through modal approaches.

The main purpose of this note is the quantitative analysis of seismic site effects using a modal approach. In the alluvial basin considered (centre of Nice), the amplification factor of weak motion is rather large. Firstly the eigenmodes of the basin are determined. They are numerous and it is generally difficult to detect those corresponding to strong site amplification. Considering the modal superposition method, the analysis of modal excitation factor and effective modal mass values gives interesting results about the main amplification frequencies of the basin. The use of effective modal mass in the modal approach allows the quantification of the contributions of each mode in the amplification process. In the specific site considered, amplification characteristics are found to be very close in both modal and experimental approaches. (C) 2000 Academie des sciences/Editions scientifiques et medicales Elsevier SAS.