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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.

Plain Language Summary Large engineered structures, such as tall buildings, show very large amplification of vibrations at distinct frequencies (resonance modes). Measuring these vibrations allows engineers to derive information about the structure, for example, information on the stiffness and geometry of the object investigated. In this study, resonance mode analysis of ambient seismic vibration data was applied on two unstable mountain slopes that can be considered as large natural structures. We installed arrays of seismometers across the instabilities and recorded the ambient seismic vibrations for approximately 1 hr. We found that the instabilities are indeed vibrating at distinct resonance modes and that the direction of deflection is perpendicular to the large-scale fracture system. At resonance modes of higher frequencies, separate units of the instability are vibrating in opposite directions. We verified our findings using a computer model of one of the unstable slopes. The technique presented in this paper can support geologists in mapping the dominant fracture network of the unstable slope. This approach allows slopes to be mapped quickly and efficiently, only requiring a few hours of seismic data recording. In contrast, slope mapping based on displacement monitoring (e.g., Global Navigation Satellite System) often requires longer time series of up to several years.