A Comprehensive Analysis of Seismic Site Effects in the Grenoble Basin (French Alps)

The Grenoble basin, located in the French Alps, is a region of significant interest for seismic hazard assessment due to its thick sedimentary layers and surrounding high massifs, leading to 2D/3D complex wave propagation patterns. With the aim to develop suitable strategies for seismic microzonation in alpine valleys, this study focuses on the seismic response of the basin using state-of-the-art 3D simulations performed with the EFISPEC3D spectral element method code for frequencies up to 5 Hz. These simulations aim to capture the intricate interactions between geological features, including lateral heterogeneity and basin geometry, which are not considered in traditional 1D microzonation approaches.

A primary goal of this research is to compare synthetic seismic data derived from 1D and 3D models with observed data to identify the limitations of 1D approach to provide a robust estimation of the site effects. Particular attention is paid to the analysis of fundamental frequencies and seismic wave amplification. While central regions of the basin exhibit consistent fundamental frequencies across 1D and 3D models, discrepancies arise at the edges due to the presence of complex lateral heterogeneities.

The study further investigates aggravation factors such as Peak Ground Velocity (PGV), Peak Ground Acceleration (PGA), and Arias Intensity, revealing significant amplification in the central areas of the basin when using 3D models. In contrast, edge zones tend to show neutral or slightly de-amplified responses. These findings underscore the importance of incorporating 3D effects into seismic hazard assessments to improve the accuracy of microzonation strategies.

Future work aims to refine seismic hazard maps by leveraging machine learning techniques to automate the classification of zones based on response spectra and frequency-dependent amplification.