ALBERT

Description: 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 1Hz, 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.

Description: Recent earthquakes have shown that edge-generated surface waves can significantly contribute to increased damages. Most observations of edge-generated surface waves are concerning long-period surface waves propagating in large-size valleys. Since travel times of such waves between valley edges can reach several tens of seconds, they are quite easy to isolate. In small-size structures, reverberating wave trains are mixed and very dense array analysis is required for the identification of basin-induced surface wave trains. The city of Grenoble (French Alps) is located in a small-size deep alluvial valley and faces important site effects (Lebrun et al., 2001). In order to identify and quantify and quantify multidimensional site effects in this basin, a very dense array of 29 three-component seismometers over a 1-km aperture was installed within the city. The wave-field complexity as well as the in situ noise characteristics (colored/correlated noise and low signal-to-noise ratio) led us to develop a procedure based on time-frequency coherence of signal waveforms and the multiple signal classification (MUSIC) (Schmidt, 1981) algorithm to identify the main energetic contributions crossing the array. Next, the nature and energy of waves were estimated using some properties of the analytical three-component covariance matrix. Careful methodological investigations were performed in order to better understand and quantify the effects of site constraints on the estimation of wave parameters with the MUSIC technique. Simulations outline the ability of array antennas first to handle difficult scenarios involving multiple, nonstationary, and correlated propagating phases and second to estimate the polarization and energy of waves. The velocity estimation is shown to be much more unstable than backazimuth estimation, and a low signal-to-noise ratio introduces some variation in estimates. Finally, considering the very large number of identified waves, a statistical view of final estimates is suggested for improving the reliability of analysis. In an accompanying article (Cornou et al., 2003), we use this method to investigate the entire wave field of seismic events recorded by the array in order to isolate basin-induced waves.