Settled on a deep sediment-filled valley, the city of Grenoble (French Alps) faces important site effects: large amplification and significant duration increase of ground motion, even for moderate-size events. In order to study multidimensional site effects, a very dense array composed of 29 three-component seismometers over a 1-km aperture was operated during spring 1999 in the center of the city. A total of 18 events (6 local, 4 regional, and 8 teleseismic) with an acceptable signal-to-noise ratio could be recorded over a 4-month period. The complexity of the wave field and in situ seismic noise constraints led us to develop a procedure based on time-frequency coherence and the multiple signal classification algorithm to identify and characterize wave arrivals (Cornou et al., 2003). Applying the procedure to the 18 records, it is clearly indicated that ground motion inside the valley is dominated by basin-edge-induced waves that carry 4 times more energy than the direct wave field, regardless of the type of event considered. In addition, the basin-induced wave field is composed of 60% Rayleigh waves and 40% Love waves when considering energy carried by the three components. If one considers only the energy of horizontal components, this proportion is 50% Rayleigh waves and 50% Love waves. The diffraction phenomena are mostly constrained by the 3D structure of the basin, regardless of the azimuth of the event. A study of the relative contribution of 1D and 2D/3D effects on recorded ground motion suggests, at least at frequencies below 1 Hz, that the difference between the standard spectral ratio and 1D transfer function, or possibly the horizontal-to-vertical ratio (receiver function and Nakamura estimates) might be due mainly to laterally propagating waves.

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