On the consistency of site amplification estimates from empirical and numerical methods in Italy

The reliable estimation of site amplification is crucial for seismic hazard and risk assessment. Although the amplification estimates are traditionally site-specific, recent efforts have focused on developing site amplification models for large areas. To this aim, we compiled existing site amplification factor estimates in Italy derived from different empirical methods. We then employed goodness-of-fit metrics and statistical tests to assess their mutual consistency and their agreement with predictions from an available 1D simulation-based model. We found a high correlation between the period-dependent site amplification factor estimates, suggesting that empirical methods consistently measure the relative amplification levels of different sites. Systematic differences among empirical estimates, observed particularly in Central Italy, are primarily due to the choice of the reference ground motion and can be corrected for by applying a simple scaling factor to a common reference ground motion level, thereby confirming the robustness of empirical estimates. Conversely, the comparison between empirical data and 1D simulations revealed a poor correlation, with the latter failing to reproduce the full range of observed amplification. For rock and stiff sites, 1D simulations tend to overestimate empirical amplification. In contrast, for a smaller fraction of soft-soil sites (e.g. within large basins), 1D simulations underestimate empirical amplification, likely due to their inability to model complex 2D/3D effects. These results suggest that the development of regional or national site-amplification models should prioritize empirical site amplification estimates.