Instrumental evidence of normal mode rock slope vibration

Abstract

S U M M A R YA unique field experiment was performed to constrain the seismic response of a large, potentially unstable rock slope in the southern Swiss Alps. Small-aperture seismic arrays were deployed to record ambient vibrations both inside and outside of the mapped instability boundary. The recordings were analysed by means of the high-resolution f-k method, site-to-reference spectral ratios and time-frequency dependent polarization analysis. All three methods indicated that the wavefield within the potentially unstable rock mass is dominated by normal mode motion (standing waves) rather than horizontal propagation of seismic waves. Both fundamental frequency and relative amplification could be recovered from ambient noise data. The observed amplification is strongly directional, and the maximum amplification is oriented perpendicular to open tension cracks mapped at the ground surface. Our results highlight site response characteristics relevant for analysis of earthquake-triggered rock slope failures.

Combining Seismic Noise Techniques for Landslide Characterization

“…Similar to what is observed across sediment-filled valleys in numerical simulations (e.g., Bouchon 1985, Frischknecht andWagner 2004) and in real data (e.g., Tucker and King 1984, Roten et al 2006), we expect the resonance frequency for such a normal mode resonance to be approximately equal over the entire extent of the landslide body, regardless of its thickness at each site, alike Burjanek et al (2012). Using classical 1D transfer function theory and the obtained velocity model (e.g., for S13 and S20, see Figure 6) one might pre-estimate that a thickness of the lowvelocity body of around 30 to 35 m will correspond to a fundamental frequency of around 2.5 Hz.…”

“…Several cracks can be observed at the surface, in particular near S3 of array 2. Such fractures, having preferred orientation perpendicular to the highest slope directions, might generate even stronger amplification effects (Burjanek et al 2012) and probably enhance instability by a local reduction of the shear strength within the upper layer.…”

“…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. Renalier et al (2010) correlated seismic noise to resolve 2D profiles of the shear-wave velocity structure of a landslide composed of clayey soil deposits.…”

Seismic site response of unstable steep slope using noise measurements: the case study of Xemxija Bay area, Malta

“…In our study, a tendency seems evident for the entire landslide body to generally vibrate with a northeast azimuth and, accordingly, during a strong earthquake the ground motion would be amplified in this direction. Studies of Burjánek et al (2010Burjánek et al ( , 2012 point out that the ambient noise polarization is at about a 90-degree angle to the observed fractures which are perpendicular to the sliding direction. In the present study the polarization angle is parallel to the opening cracks, which appears in contrast to the above mentioned results.…”

“…Recordings of ambient noise and the use of the HVSR technique have recently had widespread use in studying landslides (e.g. Del Gaudio et al, 2008;Burjánek et al, 2010;Del Gaudio and Wasowski 2011;Burjánek et al, 2012).…”