Contribution of Dense Array Analysis to the Identification and Quantification of Basin-Edge-Induced Waves, Part II: Application to Grenoble Basin (French Alps)

Abstract

Settled on a deep sediment-filled valley, the city of Grenoble (French Alps) faces important site effects: large amplification and significant duration in- crease of ground motion, even for moderate-size events. In order to study multidi- mensional site effects, a very dense array composed of 29 three-component seis- mometers 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 iden- tify 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 domi- nated 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 consid- ering 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 esti- mates) might be due mainly to laterally propagating waves.