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.
Ambient Vibration Modal Analysis of Natural Rock Towers and Fins
Abstract The dynamic properties of freestanding rock towers are important inputs for seismic stability and vibration hazard assessments, however data describing the natural frequencies, mode shapes, and damping ratios of these landforms remain rare. We measured the ambient vibration of 14 sandstone and conglomerate rock towers and fins in Utah, United States, using broadband seismometers and nodal geophones. Fundamental frequencies vary between 0.8 and 15 Hz—inversely with tower height—and generally exhibit subhorizontal modal vectors oriented parallel to the minimum tower width. Modal damping ratios are low across all features, between 0.6% and 2.2%. We reproduced measured modal attributes in 3D numerical eigenfrequency models for 10 of the 14 landforms, showing that the fundamental mode of these features is full-height bending akin to a cantilever. Fin-like landforms commonly have a torsional second mode whereas tower-like features have a second full-height bending mode subperpendicular to the fundamental. In line with beam theory predictions, our data confirm that fundamental frequencies scale with the ratio of a tower’s width to its squared height. Compiled data from 18 other sites support our results, and taken together, provide guidance for estimating the modal properties of rock towers required for vibration risk assessment and paleoseismic shaking intensity analysis in different settings.
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- Award ID(s):
- 1831283
- NSF-PAR ID:
- 10378599
- Date Published:
- Journal Name:
- Seismological Research Letters
- Volume:
- 93
- Issue:
- 3
- ISSN:
- 0895-0695
- Page Range / eLocation ID:
- 1777 to 1786
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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- Free Publicly Accessible Full Text
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1785/0220210325
