This paper presents a detailed study of the response of an alluvial valley in the Alps (France). The structure (Ubaye Valley) was chosen 1) for its moderate dimensions (500 m wide and 65 m thick), which allow an accurate determination of the deposit characteristics, and 2) for the relatively high seismicity of the region.

The study included the deployment of five temporary seismological stations, a seismic survey of the valley for determining the dynamic properties and the geometry of the soft deposits, and 1D and 2D numerical modeling for incident SH and SV waves.

Comparisons between observed and computed amplifications show a good agreement for frequencies lower than 10 Hz and the particular direction of arrival of the input motions corresponding to the 2D case. When the earthquake sources are not located in the continuation of the line of seismometers, the spectral ratios exhibit a great variability between different groups of similar events. These observations indicate the azimuthal dependence of the site response and the need for 3D modeling.

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