Monitoring the changing seismic site response of a fast-moving rockslide (Brienz/Brinzauls, Switzerland)

Abstract

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.

Coevolution of Rock Slope Instability Damage and Resonance Frequencies From Distinct‐Element Modeling

“…2 of 19 frequency values, seismic parameters including frequency-dependent polarization azimuth and damping (e.g., Geimer et al, 2022) can be used to detect internal structural or material changes during stability monitoring (e.g., Häusler et al, 2022).…”

“…Field investigations have revealed resonance changes across different landslide compartments or zones due to material characteristics, geometry, and fracture patterns (Del Gaudio et al, 2013;Galea et al, 2014;Iannucci et al, 2018;Kleinbrod et al, 2019), providing indications of resonance changes expected with instability evolution. For example, strong spectral amplification and directivity are frequently observed in the presence of steeply dipping open cracks, reflecting normal mode vibration of the main landslide body at a fundamental (or first resonance; f 1 ) frequency and vibration of secondary volumes or sub-blocks at higher frequencies (Burjánek et al, 2010(Burjánek et al, , 2012(Burjánek et al, , 2018Häusler et al, 2019Häusler et al, , 2022Moore et al, 2011).…”

“…In the sliding case, frequency changes were detectable due to open cracks created via internal failure; however, the overall resonance response was likely limited by the failure mode. Furthermore, fundamental normal mode resonance of large, deep-seated failures may be weak or difficult to distinguish, and spectral peaks corresponding to sub-volumes in highly fractured areas may dominate the seismic response (Häusler et al, 2021(Häusler et al, , 2022Moore, Geimer, Burjánek, et al, 2018). Ultimately, resonance monitoring for damage detection is well-suited for toppling landslides and sliding failures involving tension crack development via internal fracture but may be limited at sites without coherent sub-vertical fractures or for complex deep-seated instabilities.…”

“…Demonstrated decreases in resonance frequencies before the failure of a rock column (Lévy et al., 2010) and a rock block collapse test (Taruselli et al., 2020), in addition to results of a study showing an increase in resonance frequencies of an unstable rock compartment after bolting reinforcement (Bottelin et al., 2017), highlight ambient vibration measurements as a valuable slope stability monitoring technique. Although numerous additional field‐based studies have attempted to capture frequency changes associated with landslide destabilization, often only environmentally driven recoverable changes are recorded (Bottelin, Lévy, et al., 2013; Burjánek et al., 2018; Colombero et al., 2018, 2021; Dietze et al., 2021; Guillemot et al., 2022; Häusler et al., 2021, 2022; Weber et al., 2018). In addition to resonance frequency values, seismic parameters including frequency‐dependent polarization azimuth and damping (e.g., Geimer et al., 2022) can be used to detect internal structural or material changes during stability monitoring (e.g., Häusler et al., 2022).…”

“…Although numerous additional field‐based studies have attempted to capture frequency changes associated with landslide destabilization, often only environmentally driven recoverable changes are recorded (Bottelin, Lévy, et al., 2013; Burjánek et al., 2018; Colombero et al., 2018, 2021; Dietze et al., 2021; Guillemot et al., 2022; Häusler et al., 2021, 2022; Weber et al., 2018). In addition to resonance frequency values, seismic parameters including frequency‐dependent polarization azimuth and damping (e.g., Geimer et al., 2022) can be used to detect internal structural or material changes during stability monitoring (e.g., Häusler et al., 2022).…”