Abstract Thousands of rock arches are situated within the central Colorado Plateau—a region experiencing small- to moderate-magnitude contemporary seismicity. Recent anthropogenic activity has substantially increased the seismicity rate in some areas, raising questions about the potential for vibration damage of natural arches, many of which have high cultural value. However, predictions of the vibration response and potential for damage at a given site are limited by a lack of data describing spectral amplification of ground motion on these landforms. We analyzed 13 sandstone arches in Utah, computing site-to-reference spectral amplitude ratios from continuous ambient seismic data, and compared these to spectral ratios during earthquakes and teleseismic activity. We found peak ground velocities on arches at their dominant natural modes (in the range of 2–20 Hz) are ∼20–180 times the velocity on adjacent bedrock, due to amplification arising from slender geometry and low modal damping (0.8%–2.7%). Ambient spectral ratios are generally 1.2–2.0 times the coseismic spectral ratios. Because arches experience highly amplified ground motion, the range of earthquakes considered potentially damaging may need to be revised to include lower-magnitude events. Our results have implications for conservation management of these and other culturally valuable landforms.
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Contribution of anthropogenic vibration sources to crack growth in natural rock arches
Natural arches are culturally valued rock landforms common in sedimentary rocks of the Colorado Plateau and additionally occur broadly around the world. Recent notable collapses of some of these landforms have highlighted the need to better understand the mechanics of their failure. While environmentally driven weathering has been the focus of most previous studies of arch collapse, comparably little attention has been given to anthropogenic vibration sources and how these often slight- to moderate-magnitude shaking events might steadily weaken arches over time. We collected 12–15 months of continuous ambient vibration data from arches and nearby bedrock in both anthropogenically ‘noisy’ and ‘quiet’ locations and used these datasets to develop an annual model of arch peak ground velocity based on magnitude-cumulative frequency distributions. Working from these models, we added vibration events of varying magnitude or frequency of occurrence, informed by field data, imitating arch vibration in response to different anthropogenic activities such as helicopter flights or induced earthquakes. We then applied subcritical fracture mechanics principles to predict annual crack growth rates in an idealized arch under these different vibration conditions. Our results demonstrate that in a single year, cracks grow minimally longer (∼1%) in ‘noisy’ environments than in areas not experiencing anthropogenic vibration energy. Few (1+) 30-s moderate-magnitude events (∼15 mm/s) or many (>37,000) 30-s low-magnitude events (∼2 mm/s) cause markedly increased crack growth. Our approach provides a valuable new framework for assessing the range and frequency of occurrence of vibrations experienced by an arch, and for predicting arch damage. Our results, in turn, yield important new outputs applicable in support of conservation management of these and similar landforms world-wide under exposure to a range of human-induced vibration activity.
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- Award ID(s):
- 1831283
- NSF-PAR ID:
- 10378603
- Date Published:
- Journal Name:
- Frontiers in Earth Science
- Volume:
- 10
- ISSN:
- 2296-6463
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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Abstract 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.
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- Free Publicly Accessible Full Text
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.3389/feart.2022.1035652
