QUANTIFICATION OF SURFACE WAVES IN ALLUVIAL BASIN: SIMULATIONS FOR CANONICAL AND REALISTIC CASES

Seismic waves propagating in alluvial deposits may generate surface waves due to lateral heterogeneities. Such basin edge generated surface waves strengthen the amplification process and lead to long period ground motions. These long period excitations may be detrimental for tall or large structures. This paper focuses on the quantification of basin edge generated surface waves by time-frequency analysis. Numerical simulations for canonical and realistic configurations lead to various seismograms involving a significant amount of surface waves. The normalized inner product proposed by Meza-Fajardo et al. (2015) allows to identify and quantify the contribution of surface waves in the total computed ground motion. For canonical basins, the influence of the basin edge slope and of the basin/bedrock impedance contrast on the amount of surface waves is characterized. For an actual configuration in the city of Rome, the highly heterogeneous deposit leads to strong surface waves of various origins: basin edge generated wavefield, sub-basin effects for non-smooth deposit geometries, local strong heterogeneities. It is thus very important to identify these various contributions in order to characterize the basin effects and the long period seismic ground motions (azimuth, amplitude, time window). This work is part of the Modulate research project funded by French National Research Agency (ANR).