Comprehensive analysis of HVSR for Italian sites: variability, Vs30 prediction, and peak frequency-based site term adjustment

This study presents a comprehensive investigation of the variability and application of the horizontal-to-vertical spectral ratio (HVSR) for site characterization in Italy. Using data from 296 stations with HVSR computed in both the Fourier amplitude spectrum (FAS) and spectral acceleration (SA) domains (HVSR FAS and HVSR SA , respectively), we found that HVSR peak frequencies ( f p ) could be identified at a substantially higher proportion of sites from HVSR FAS than from HVSR SA (76% versus 52%). Among the 149 sites with f p identified from both methods, approximately 8% exhibited significant differences in f p , and the corresponding peak amplitudes ( A p ) from HVSR SA were, on average, 0.798 times those from HVSR FAS . Based on the extracted HVSR parameters, six empirical models were developed to estimate the measured time-averaged shear-wave velocity in the upper 30 m ( V s30 ). The proposed V s30 prediction models are applicable only to stations with identifiable HVSR peaks and were internally validated using an independent testing dataset comprising 30 stations. Among the six models, the one incorporating both f p and A p from HVSR FAS provided the best performance. All six proposed models achieved comparable or lower errors than most existing models, including the slope-based approach, in estimating measured V s30 . When applied to an Italian ground motion prediction equation (GMPE), the use of the proposed HVSR-based V s30 reduced the mean absolute error ( MAE ) by an average of 1.7%, but increased intra-event standard deviation ( ϕ ) and site-to-site standard deviation ( ϕ s 2 s ) by average values of 1.5% and 3.0%, respectively, compared with the slope-based V s30 . These results indicate that, although the proposed V s30 is useful as a proxy, its direct substitution into an existing GMPE may not reduce aleatory variability without recalibration. In addition, an f p -based adjustment term was developed for the Italian GMPE, which reduced the MAE , ϕ , and ϕ s 2 s by average values of 1.1%, 2.6%, and 5.2%, respectively.