Structural Characterization of a Toppling Rock Slab From Array‐Based Ambient Vibration Measurements and Numerical Modal Analysis

Accurate assessments of the internal structure and boundary conditions of unstable rock slopes are imperative for evaluating landslide hazard scenarios. However, instability characterization at depth remains challenging and is often limited by costly or invasive subsurface investigations. Here, we develop a new approach coupling array‐based ambient vibration modal analysis and numerical modeling to improve structural characterization of rock slope instabilities at depth. We used ambient noise cross‐correlation on 4 hr of seismic data recorded by an array of 30 nodal geophones at a 500‐m‐long toppling rock slab in Utah, USA to identify modal frequencies between 0.8 and 3.5 Hz and derive modal displacements. We show that transverse and longitudinal bending modes span the length of the instability, indicating an interconnected slab. Statistical comparison of field results with outputs from >1,000 finite element models with varying boundary conditions showed that the instability depth varies between 40–70 and 10–20 m in the middle and lateral regions, respectively. Our approach yields new information on the structural conditions of rock cliff and column instabilities at depth, which is not easily obtained by other means but is imperative for change detection monitoring and improved hazard assessments. Rock slope failures constitute a major hazard in areas with steep terrain and can endanger human lives and damage infrastructure. An important step in minimizing the hazard from unstable rock slopes is obtaining a detailed picture of the structure and geometry of an instability, including size, material properties, and the manner in which failure is likely. Characterizing the structure of landslides below the surface, however, remains challenging and is often costly or requires intrusive measurements. We developed a new approach that combines surface‐based measurements of the natural vibrations of an unstable rock cliff with numerical modeling to improve estimates of the instability geometry. We measured the vibrational properties of a large unstable rock slab in southern Utah, USA and compared the results of our field data analysis with >1,000 model iterations to narrow down the possible geometries of the instability below the surface. This technique improved our assessment of the instability size and structure, and has the potential to be a valuable tool for similar evaluations at other unstable rock slopes. We combine ambient vibration modal analysis and numerical modeling to assess structural conditions of a toppling rock slab in Utah, USAMode shapes derived from geophone array data are used to calibrate numerical eigenfrequency models and invert for boundary conditionsOur approach can be applied in other settings to enhance characterization of unstable rock slopes in support of improved hazard assessments We combine ambient vibration modal analysis and numerical modeling to assess structural conditions of a toppling rock slab in Utah, USA Mode shapes derived from geophone array data are used to calibrate numerical eigenfrequency models and invert for boundary conditions Our approach can be applied in other settings to enhance characterization of unstable rock slopes in support of improved hazard assessments