The dynamic properties of freestanding rock landforms are a function of fundamental material and mechanical parameters, facilitating non-invasive 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. Dense-array cross-correlation results from an additional arch validate 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.
