Ambient vibration analysis of an unstable mountain slope

A field experiment with small aperture seismic arrays was performed on the unstable rock slope above the village of Randa in the southern Swiss Alps. The aim of this experiment was to constrain the seismic response of a potential future rockslide using ambient vibration recordings. Weak seismic events were identified on the recordings and site-to-reference spectral ratios were calculated using a reference site located on the stable part of the slope. Spectral ratios of up to 30 were observed at sites located within the unstable portion of the slope. A strong variation of spectral ratios with azimuth indicates a directional site effect. Neither amplification nor directionality were observed at sites located in the stable part of the slope. Furthermore, time-frequency polarization analysis of the ambient noise was performed to provide robust estimates of frequency dependent directions of the maximum polarization. It was found that the unstable part of the slope vibrates within a narrow range of directions (130 +/- 10 degrees) for the frequency range centred around 5 Hz. The polarization directions estimated from ambient seismic vibrations are in good agreement with the deformation directions obtained by geodetic and in situ measurements. No directionality of ambient vibrations was observed at sites within the stable part of the slope.