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National audienceAmbient vibration techniques such as the H/V method may have the potential to significantly contribute to site effect evaluation, particularly in urban areas. Previous studies interpret the so-called Nakamura's technique in relation to the ellipticity ratio of Rayleigh waves, which, for a high enough impedance contrast, exhibits a pronounced peak close to the fundamental S-wave resonance frequency. Within the European SESAME project (Site EffectS assessment using AMbient Excitations) this interpretation has been tested through noise numerical simulation under well-controlled conditions in terms of source type and distribution and propagation structure. We will present simulations for a simple realistic site (one sedimentary layer over bedrock) characterized by a rather high impedance contrast and low quality factor. Careful H/V and array analysis on these noise synthetics allow an in-depth investigation of the link between H/V ratio peaks and the noise wavefield composition for the soil model considered here: (1) when sources are near (4 to 50 times the layer thickness) and surficial, H/V curves exhibit one single peak, while the array analysis shows that the wavefield is dominated by Rayleigh waves; (2) when sources are distant (more than 50 times the layer thickness) and located inside the sedimentary layer, two peaks show up on the H/V curve, while the array analysis indicates both Rayleigh waves and strong S head waves; the first peak is due to both fundamental Rayleigh waves and resonance of head S waves, the second is only due to the resonance of head S waves; (3) when sources are deep (located inside the bedrock), whatever their distance, H/V ratio exhibit peaks at the fundamental and harmonic resonance frequencies, while array analyses indicate only non-dispersive body waves; the H/V is thus simply due to multiple reflections of S waves within the layer. Therefore, considering that experimental H/V ratio (i.e. derived from actual noise measured in the field) exhibit in most cases only one peak, we conclude that H/V ratio is (1) mainly controlled by local surface sources, (2) mainly due to the ellipticity of the fundamental Rayleigh waves. Then the amplitude of H/V peak is not able to give a good estimate of site amplification facto

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International audienceThe horizontal-to-vertical (H/V) method has the potential to significantly contribute to site effects evaluation, in particular in urban areas. Within the European project, site effects assessment using ambient excitations (SESAME), we investigated the nature of ambient seismic noise in order to assess the reliability of this method. Through 1D seismic noise modeling, we simulated ambient noise for a set of various horizontally stratified structures by computing efficiently the displacement and stress of dynamic Green's functions for a viscoelastic-layered half-space. We performed array analysis using the conventional semblance-based frequence-wavenumber method and the three-component modified spatial autocorrelation method on both vertical and horizontal components and estimated the contribution of different seismic waves (body/surface waves, Rayleigh/Love waves) at the H/V peak frequency. We show that the very common assumption that almost all the ambient noise energy would be carried by fundamental-mode Rayleigh waves is not justified. The relative proportion of different wave types depends on site conditions, and especially on the impedance contrast. For the 1D horizontally layered structures presented here, the H/V peak frequency always provides a good estimate of the fundamental resonance frequency whatever the H/V peak origin (Rayleigh wave ellipticity, Airy phase of Love waves, S-wave resonance). We also infer that the relative proportion of Love waves in ambient noise controls the amplitude of the H/V peak

S U M M A R YWe performed extensive ambient vibration measurements in the basin of Santiago de Chile (Chile), and we look for testing the reliability of the horizontal-to-vertical amplitude spectra ratio method (H/V ) as a tool to provide qualitative and quantitative information of site effects in complex geological media. The interpretation of the H/V data was carried out conformably to the SESAME project consensus criteria and outlines three major patterns: (1) clear peaked H/V curves related to sharp underground velocity contrast; (2) H/V peak of low amplitude and flat curve related to weak contrast and (3) broad H/V peak indicating the presence of strong lateral variations of underground structure. H/V measurements, however, reveal a discrepancy between the computed soil resonance frequencies and the expected building resonance, therefore not leading to a straight interpretation of the intensity distribution derived from observed damage to one storey houses in Santiago after the 1985 Valparaiso earthquake. Indeed, the H/V technique mostly maps the first fundamental frequency; however, it fails to show higher resonance modes. In the case of the city of Santiago, this method works well for assessing the seismic hazard for high-rise buildings, but is questionable for smaller structures as is the case of a great percentage of constructions in the city.