Mapping site amplification with the dense recording of ambient vibration for the city of Lucerne (Switzerland): comparison between two approaches

Reliable site-specific amplification information can be retrieved using earthquake-based methods that involve the deployment of a permanent (or temporary) network of seismic recording stations. Such an endeavour may need to operate for years, especially within regions of high risk but low recurrence rates in seismic activity. Hence, time- and cost-effective approaches using ambient vibrations are gaining popularity. Among such techniques, the canonical correlation approach (CC) collates measured empirical amplification with its indicator computed from ambient vibrations (i.e. horizontal-to-vertical spectral ratios) for a training set of seismic stations, to predict site response at locations without earthquake recordings. Another method, the hybrid standard spectral ratio method (SSRh) takes advantage of simultaneous recordings of ambient vibrations that are adjusted using earthquake ground motion data using a limited number of instrumented sites to estimate local seismic soil response. We apply both methods in the Lucerne area (Switzerland), which is located on a soft sedimentary basin, and obtain consistent results that are comparable to amplification estimates derived solely from earthquake ground motion data. These results show significant linear amplification factors (8–10 or more) at the fundamental frequency of resonance of the sediments (0.8–2 Hz). However, both techniques show systematic differences in the spatial and frequency domains. The CC method tends to underestimate the amplification at the fundamental frequency, while the SSRh technique predicts higher amplification in the centre of the basin and lower amplification at the basin edges in comparison to the CC approach. The study discusses the impact of the limitations in the completeness of the calibration dataset, and variability introduced by the choice of the shear wave velocity model of the shallow subsurface and inelastic behaviour treatment for the CC method, as well as the influence of the measurement setup for the SSRh method.