The dynamic properties of freestanding rock landforms are a function of fundamental material and mechanical parameters, facilitating noninvasive vibration-based structural assessment. Characterization of resonant frequencies, mode shapes, and damping ratios, however, can be challenging at culturally sensitive geologic features, such as rock arches, where physical access is limited. Using sparse ambient vibration measurements, we describe three resonant modes between 1 and 40 Hz for 17 natural arches in Utah spanning a range of lengths from 3-88 m. Modal polarization data are evaluated to combine field observations with 3-D numerical models. We find outcrop-scale elastic moduli vary from 0.8 to 8.0 GPa, correlated with diagenetic processes and identify low damping at all sites. Correlation of dense-array measurements from one arch validates predictions of simple bending modes and fixed boundary conditions. Our results establish use of sparse ambient resonance measurements for structural assessment and monitoring of arches and similar freestanding geologic features worldwide.Plain Language Summary Natural rock arches vibrate under ambient conditions with a unique set of frequencies controlled by geometry, host material, and interactions with nearby bedrock. Recent rockfall events at well-known arches in Utah have highlighted the need to develop noninvasive assessment methods to better understand how these sensitive landforms evolve. To reduce site impacts, we employed limited instrumentation to measure ambient vibrations of 17 arches across Utah for identification of resonant frequencies. We combine direct observations with predictive numerical models to visualize resonant mode shapes and describe the controlling material properties and structural boundaries. In defining the first three modes of each site, we are able to characterize dynamic properties at arches encompassing several geologic formations and a range of length scales. These results establish a versatile method for structural evaluations of arches and other significant freestanding geologic features.

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The resonance characteristics of freestanding structures contain information about bulk material and structural properties, providing a noninvasive and passive means to assess changing stability by monitoring modal parameters over time. However, efforts to identify precursory signs of damage or ultimate collapse using such parameters are often complicated by structural and material sensitivities to changing environmental conditions, such as temperature and moisture (e.g.

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

We acquired a unique ambient vibration dataset from Castleton Tower, a 120 m high bedrock monolith located near Moab, Utah, to resolve dynamic and material properties of the landform. We identified the first two resonant modes at 0.8 and 1.0 Hz, which consist of mutually perpendicular, linearly polarized horizontal ground motion at the top of the tower. Damping ratios for these modes were low at ∼1%. We successfully reproduced field data in 3D numerical eigenfrequency simulation implementing a Young’s modulus of 7 GPa, a value ∼30% lower than measured on core samples. Our analysis confirms that modal deformation at the first resonant frequencies closely resembles that of a cantilever beam. The outcome is that with basic estimates of geometry and material properties, the resonant frequencies of other freestanding rock monoliths can be estimated a priori. Such estimates are crucial to evaluate the response of rock towers to external vibration inputs.