Spectral analysis and correlation of ambient seismic noise. The case study of Madonna del Sasso (NW Italy)

Ambient vibrations recorded on potentially-unstable rock slopes show that the temporal variations in the spectral content and in the correlation of seismic noise can be related to both reversible and irreversible changes within the rock mass. In this work, we analyzed the seismic recordings acquired at the potentiallyunstable granitic cliff of Madonna del Sasso (NW Italy) from October 2013 to November 2014. The spectral content of noise systematically highlighted clear energy peaks at specific frequencies on the most unstable sector, interpreted as resonant frequencies of the investigated volume. Horizontal ground motion at the fundamental frequency was moreover found to be orthogonal to the main fractures observed at the site and consequently parallel to the potential direction of collapse. Cross-correlation was computed between the recordings of the sensors placed in the prone-to-fall compartment and a stable reference station. Both the temporal variations of the resonant frequencies and the results of cross-correlation showed seasonal reversible variations related to temperature fluctuations. No irreversible changes, resulting from damage processes within the rock mass, were detected during the monitored period.