Contribution of Dense Array Analysis to the Identification and Quantification of Basin-EdgeInduced Waves, Part I: Methodology

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 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, nonstationnary, 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.