Most seismic codes adopt as a key quantitative parameter the average shear wave velocity in the first 30 m of subsoil, commonly calling it Vs30. Estimates of Vs30 are therefore required in most countries for microzonation both at large-scale as well as at the scale of the single buildings. We propose a fast and inexpensive technique to measure the Vs30 based on the horizontal to vertical spectral ratio (H/V) of microtremor recorded at a single station. The experimental H/V is fitted with a synthetic curve using as a constraint the thickness of the most superficial layer of the subsoil, which is always independently known in the geotechnical practice for building design approval. The fitting procedure consists of three steps: (1) identify the depth of the first shallow stratigraphic horizon from geotechnical data, (2) identify its corresponding H/V marker and (3) use it as a constraint to fit the experimental H/V with the synthetic one. The synthetic H/V curve is calculated by assuming a tremor wavefield with Rayleigh and Love waves in the fundamental mode in a stratified 1-D soil model. A validity check of this technique has been performed on a variety of geological settings in Northern Italy. The validity check compares: (1) the theoretical Rayleigh wave phase velocity dispersion curves calculated for the models derived from the H/V fits with the experimental curves measured with ESAC and ReMi array surveys, (2) the Vs30 estimates obtained with the proposed technique with those obtained at the same sites by using the latter array techniques. The inferred stratigraphy has been also compared with the geological knowledge. The proposed technique is found capable to provide, at a small fraction of the costs and survey times, Vs30 estimates coherent with those measured by ESAC and ReMi. Furthermore, the proposed technique is also found to be more informative than array techniques, allowing to detect deviations from 1-D subsoil geometry over lengths of a few meters, the correctness of which was confirmed by direct drilling.
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Castellaro, S., Mulargia, F. (2009). Estimates of Vs30 Based on Constrained H/V Ratio Measurements Alone. In: Mucciarelli, M., Herak, M., Cassidy, J. (eds) Increasing Seismic Safety by Combining Engineering Technologies and Seismological Data. NATO Science for Peace and Security Series C: Environmental Security. Springer, Dordrecht. https://doi.org/10.1007/978-1-4020-9196-4_7
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