Abstract

Most seismic codes adopt the average shear-wave velocity as a key parameter in the first 30 m of subsoil (VS30). Estimates of VS30 are therefore required for both large- and small-scale seismic microzonation. We propose a technique to measure the VS30 based on the horizontal to vertical spectral ratio (H/V) of microtremor recorded at a single station. The H/V is fitted with a synthetic curve using the independently known thickness of a superficial layer of the subsoil as a constraint. The proposed procedure consists of three steps: (1) identify the depth of a shallow stratigraphic horizon from independent geotechnical data, (2) identify its corresponding H/V marker, and (3) use it as a constraint to fit the experimental H/V curve with the theoretical one. The synthetic H/V curve is calculated by assuming a stratified one-dimensional (1D) soil model and a tremor wave field from distant random sources that are composed of Rayleigh and Love waves. This technique has been validated on different geological settings following a triple check procedure: (1) the theoretical Rayleigh-wave phase velocity dispersion curves calculated for the model derived from the H/V fit were compared to the experimental curves measured with the 2D extended spatial autocorrelation (ESAC) method and the 1D refraction microtremor (ReMi) array surveys; (2) the VS profiles and (3) the VS30 estimates obtained by the proposed technique were compared to those obtained by using the latter array techniques. Finally, the inferred stratigraphy was compared to the geological model. The proposed technique is not meant to provide accurate VS profiles but has the potential to provide VS30 estimates that are coherent with those measured through ESAC and ReMi and to satisfy law requirements. Furthermore, the H/V is found to be capable in detecting deviations from 1D subsoil geometry over the length of a few meters, the correctness of which was confirmed by direct drilling.

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