Tower in Motion: Resonance Mode Analysis of Devils Tower, Wyoming, USA

Devils Tower, Wyoming, is a 265-m-high cylindrical rock tower of phonolite porphyry with deep cultural significance and is the site of America’s first national monument. We deployed a seismometer on the summit of Devils Tower for
21 h in October 2024, in addition to two identical sensors at the base, and used ambient vibration modal analysis techniques to identify resonance modes. Close axial symmetry of the tower gives rise to similar frequencies for the first two modes at 1.1 and 1.2 Hz. We then performed 3-D numerical modeling to predict modal deformation fields: the first two predicted modes are full-height swaying of the tower in orthogonal directions, matching resonance frequencies from field data, followed by a third, torsional mode. The model derived a calibrated global Young’s modulus for the tower of 8 GPa, which is approximately seven times lower than that measured from intact rock testing due to the added compliance of joints. Our results contribute to a growing understanding of the structural dynamics of freestanding rock landforms with different sizes, geometry, and composition, and more generally to engaging public interest in geologic features of cultural heritage sites.