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 accelerometer monitoring of slopes appears impractical, it is possible to derive information relevant for stability conditions from passive seismic techniques based on the acquisition 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 recordings. An analysis of azimuthal variation of H/V can reveal directional resonance phenomena affecting slope areas, providing the frequency and direction of maximum 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 recordings, wave trains that can be reliably identified as Rayleigh waves. A new promising technique is based on the identification of instantaneous polarisation properties from analytical signal transformation. The study of slope dynamic response to shaking can also benefit from the analysis of cross-correlation among simultaneous noise recordings. This type of analysis provides dispersion curves for Rayleigh waves, which can then be used to constrain S-wave velocity models and, consequently, to infer mechanical properties of slope materials.