Characterization of unstable rock slopes through passive seismic measurements

In this ETHZ-funded project we analyze ambient vibration and earthquake recordings to characterize and investigate the dynamic response of unstable rock slopes. We perform systematic measurements and interpretation of ambient vibrations at known unstable rock slopes, both with single stations and array-configurations. The eigenfrequencies, eigenmodes, directivity, and amplification of ambient vibrations are identified and compared to geotechnical investigations. The interpretation of recordings targets the estimation of the potential landslide volume and is supported by numerical modeling of seismic wave propagation in fractured media. A classification scheme based on the seismic response has been introduced. Each class indicates specific properties of a rock instability. The two main classes found are the volume-controlled sites and the depth-controlled sites. The extensive database will be extended with instabilities on high-alpine permafrost locations.

Moreover, both short-term and long-term monitoring is undertaken to understand the time evolution of the slope structure. Short-term monitoring is performed at the Alpe di Roscioro (Preonzo) site which represents a unique opportunity to monitor a slope close to collapse. A second semi-permanent station will be installed on the large rock instability above Brienz (Grisons). A main goal of the monitoring is to understand the effect of weather and climate on the dynamic behavior of the rock and its stability. Long-term monitoring is based on the analysis of available past recordings from existing seismic networks, especially for stations located at or very close to steep cliffs. In a later stage of the project, the effect of earthquakes on the rock slope stability will be evaluated using numerical modelling. It will be evaluated, if ambient noise measurements can provide a direct proxy for the seismic vulnerability of rock slope instabilities. The expected results have the potential to be applied directly in hazard analysis and risk reduction measures.