We evaluate the dynamic response of the large (around five million cubic meters) unstable rock slope at Randa, Switzerland, based on ground-motion recordings during small earthquakes. Seismic measurements revealed strong polarization of the wavefield in the direction of maximum slope displacement with spectral amplification factors ∼5 within the unstable area. Results further highlighted resonant frequencies of both the large unstable rock mass and individual blocks within it. Block vibration was confirmed by phase analysis of in situ displacement measurements from two bounding tension fractures during a small earthquake. Numerical simulation using a discontinuous elastic model showed that the presence of steeply-dipping compliant fractures is crucial for recreating the observed site response. Such fractures are common in rock slope instabilities and are here shown to contribute to polarization and amplification of seismic shaking, site effects that are rarely considered for hard rock slopes and which may influence the potential for earthquake-triggered failure.

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