We performed modal analysis using frequency domain decomposition of ambient seismic vibration data collected on large rock slope instabilities. This technique enables a robust detection of resonance frequencies and provides the corresponding mode shape vectors. We applied the technique to synthetic and field data sets acquired by seismometer arrays on two rock instabilities in Switzerland. We found that, at the fundamental mode, the entire instability vibrates in‐phase with the dominant mode shape vector oriented perpendicular to dominant fracture systems. At higher frequencies, different compartments of the instability resonate antiphase. Therefore, delineating the zero crossings between the phases allows dominant fractures to be efficiently mapped. Approximately 1 hr of ambient vibration data suffices to apply the method successfully. The method also potentially detects hidden fractures that cannot be observed by geological field mapping. In addition, this approach combines classic amplification and polarization analysis into one technique, simplifying data processing efforts.

Over the past few decades, the potential of collocated measurements of 6C data (3C of translational and 3C of rotational motion) has been demonstrated in global seismology using high-sensitivity, observatory-based ring laser technology. Proposed applications of 6C seismology range from tomographic reconstruction of near-receiver structure to the reduction of nonuniqueness in seismic source inverse problems. Applications to exploration problems have so far been hampered by the lack of appropriate sensors, but several applications have been proposed and demonstrated with array-derived rotational motion estimates. With the recent availability of, for example, fiber-optic-based high-sensitivity rotational motion sensors, widespread applications of 6C techniques to exploration problems are in sight. Potential applications are based on, for example, the fact that the extended set of combined translational and rotational motion observations enables carrying out array-type processing with single-station recordings such as wavefield separation and surface-wave suppression. Furthermore, measuring the rotational component (curl) of the seismic wavefield enables direct isolation of the S-wave constituents and could significantly improve S-wave exploration. Rotational measurements provide estimates of the spatial wavefield gradient at the free surface that allow carrying out analyses such as local slowness estimation and wavefield reconstruction. Furthermore, rotational motion measurements can help to resolve wavefield infidelity introduced by seismic instruments that are not well-coupled to the ground.

SUMMARY Seismic measurements on unstable rock slopes are a complementary tool to surface displacement surveys to characterize and monitor landslides. A key parameter is seismic amplification, which tends to scale with the degree of rock mass degradation. Amplification also provides a direct measure of how the wavefield is intensified during seismic loading, eventually leading to coseismic failure. Here we present the dynamic response of the fast-moving Brienz/Brinzauls rock slope instability in Switzerland (10 $ \times $ 106 to 25 $ \times $ 106 m3), which threatens settlements and infrastructure in the area. The rockslide shows strong seismic amplification at two resonant frequencies with factors of up to 11 and wavefield polarization influenced by the local fracture network orientation. We monitored the dynamic response over a period of 30 months using ambient vibrations and regional earthquake recordings. We observed a change in wavefield polarization of up to 50°, coinciding with a rotation of the relative surface displacement vector field measured by geodetic systems, highlighting the linkage between wavefield polarization and stress field (i.e. rock mass kinematics). For the analysis of secondary, relative surface displacements, we propose a singular value filtering of the displacement field to remove the principal component of landslide motion. In addition, we found increased seismic amplification values after periods of strong precipitation, providing empirical field evidence that the local precipitation history is a key parameter for assessing the hazard of earthquake-induced slope failure.

We monitored an unstable rock slope using frequency domain decomposition modal analysis • We tracked the resonance frequency, mode shapes, and damping ratio of the fundamental and first higher mode over a period of four years • Our study shows seasonal variations of all dynamic parameters and a weak linear relationship between frequency and damping ratio Accepted ArticleThis article has been accepted for publication and undergone full peer review but has not been through the copyediting, typesetting, pagination and proofreading process, which may lead to differences between this version and the Version of Record. Please cite this article as

Assessing and mitigating hazards associated with rock slope failures requires detailed knowledge of instability geometry, material properties and boundary conditions (