Abstract
Q values of shallow materials are derived from seismic experiments in different frequency ranges. Three main measurement techniques are used: the rise-time method, the spectral ratio method and the study of surface wave attenuation. Attenuation measurements are performed in different geological site conditions. The Q values obtained at high frequency with the rise-time and spectral ratio methods are found to be lower than the values inferred from surface waves at lower frequencies. These results suggest a frequency dependence of the quality factor in surficial materials. Two sites are thoroughly investigated by geophysical prospecting. On the basis of the geometry and the dynamic characteristics of the geological formations, numerical modeling (1D and 2D) is carried out in order to compute the site response to small earthquakes or other solicitations. Comparisons between theoretical and experimental transfer functions show a good agreement if the Q values used in the computations are those measured in the same frequency range as that of the solicitation. These results stress the importance and the need of careful investigations about the frequency dependence of Q.
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