Ambient vibration analysis of an unstable mountain slope

Abstract

S U M M A R YA 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 • ) 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.

Speedy Techniques to Evaluate Seismic Site Effects in Particular Geomorphologic Conditions: Faults, Cavities, Landslides and Topographic Irregularities

“…A direct estimate of the polarization angle, for noise data, can be achieved through two different methods. The time domain method (TD) by Jurkevics (1988) and the time-frequency (TF) polarization analysis by Burjánek et al (2010 and. The results obtained through polarization techniques are quite robust since these approaches are very efficient in overcoming the bias linked to the denominator behavior that could occur in the HVNR's technique and at the same time, longer time-series are processed therefore reducing the problems that may be linked to signal-to-noise ratio.…”

“…Moreover, a direct estimate of the polarization angle by using the covariance matrix method (Jurkevics, 1988) and the time-frequency (TF) polarization method (Burjánek et al, 2010; was achieved.…”

Combining Seismic Noise Techniques for Landslide Characterization

“…it becomes polarized in one direction at one single resonance frequency for all stations on the landslide. Following Del Gaudio and Wasowski (2007) and Burjanek et al (2010), due to the peaks in the axial Fourier spectra (e.g., S4 and S11 in Figure 4), it is likely that the entire unstable landslide mass, being around 360 m long and 300 m wide, might be viewed as a volume of effectively reduced elastic moduli, analogous to the sediment infill in valleys. Consequently, this mass vibrates in this direction of freedom with a normal mode resonance frequency of around 2.5 Hz, and it is likely that ground motion during a strong event will be amplified in this direction.…”

“…Danneels et al (2008) used small seismic arrays to measure the dispersion curve of surface waves and, subsequently, inverted for two 1D shear-wave velocity profiles in the active area. Burjanek et al (2010) estimated amplification factors and identified the range of resonant frequencies for a large unstable rock mass by noise polarization analysis. Burjanek et al (2012) used small seismic arrays to identify normal mode motion (standing waves) within an unstable rock mass.…”

“…Locally enhanced or reduced amplitudes of the H/V spectral ratios as seen in Figure 3 might reflect lateral variations in material properties of the landslide body which might represent differences between slightly more stable and more unstable parts of the slope, although the transition from the stable to the unstable part is rather smooth, meaning that a clear boundary of the unstable part can hardly be mapped (Burjanek et al 2010). For example, the H/V amplitudes in the northern part of array 2 are rather high for all measurement sites (see e.g., S3 in Figure 3) and we assume that this area might represent an extended unstable area (cf.…”