Passive seismic techniques for the assessment of dynamic slope stability conditions

Slope stability analysis in seismically active areas must take into consideration the influence of site response during earthquakes. While widespread accelerome-ter monitoring of slopes appears impractical, it is possible to derive information relevant for stability conditions from passive seismic techniques based on the ac-quisition and analysis of ambient noise generated by natural and anthropic sources. A useful technique is Nakamura’s method, which consists in analysing ratios between horizontal (H) and vertical (V) component spectra of noise record-ings. An analysis of azimuthal variation of H/V can reveal directional resonance phenomena affecting slope areas, providing the frequency and direction of max-imum ground motion amplification. Determination of site response properties from seismic noise mainly depends on the identification of polarisation direction and ellipticity of Rayleigh waves. However, in ambient noise records only part of acquired signals may show coherent characteristics referable to Rayleigh waves. Thus, it is useful to develop techniques that allow selecting, within noise record-ings, wave trains that can be reliably identified as Rayleigh waves. A new promis-ing technique is based on the identification of instantaneous polarisation proper-ties from analytical signal transformation. The study of slope dynamic response to shaking can also benefit from the analysis of cross-correlation among simulta-neous noise recordings. This type of analysis provides dispersion curves for Ray-leigh waves, which can then be used to constrain S-wave velocity models and, consequently, to infer mechanical properties of slope materials.