Estimation of site-specific amplification function from quarter-wavelength velocity profile and its application to obtain local amplification maps

Quarter-wavelength (QWL) velocity refers to the S-wave velocity at a depth equal to one-fourth of the wavelength of the seismic waves. It provides valuable information about the characteristics of the subsurface material properties affecting seismic waves propagation. The Swiss Seismological Service (SED) network, with over 200 stations across different lithologies, offers a rich dataset to implement correlation between site properties and site amplification factors. The current study is based on a subset of 113 selected SED seismic stations for which shearwave velocity (Vs) profiles from geophysical measurements are available. We first meticulously analyzed and adjusted them to accurately determine the bedrock depth of the sites they are located on. Using empirical spectral modelling (ESM), we computed amplification functions from recorded earthquakes, referenced to the Swiss standard rock profile with a V S30 of 1100 m/s. Then we performed a bivariate analysis between empirical amplification functions, QWL velocity and QWL velocity contrast. The resulting coefficients are used to predict elastic amplification at frequencies between 0.5 and 10 Hz, with predictions generally being within 20 - 28% of observed values. We also developed an empirical equation relating V S30 (time-averaged Vs to 30 m depth) and kappa 0 (high-frequency attenuation parameter), to incorporate the anelastic term in the amplification. Applying our method to a 3D geophysical model of Visp, a high seismic hazard zone in Switzerland, we found it effective in predicting 1D site amplification, but noted caution in areas susceptible of 2D/3D resonance effects. A calibration function based on observed vs. predicted amplification and the frequencies of estimation normalized by the expected 2D resonance frequency improved predictions for Visp. Our study demonstrates that QWL velocity profiles offer a straightforward approach for characterizing site effects, which can be used in seismic hazard assessment to evaluate the potential amplification of ground motions.