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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Vibration of natural rock arches and towers excited by helicopter-sourced infrasound
Abstract. Helicopters emit high-power infrasound in a frequency range thatcan coincide with the natural frequencies of rock landforms. While a singleprevious study demonstrated that close-proximity helicopter flight was ableto excite potentially damaging vibration of rock pinnacles, the effects on abroader range of landforms remain unknown. We performed a series ofcontrolled flights at seven sandstone arches and towers in Utah, USA,recording their vibration response to helicopter-sourced infrasound. Wefound that landform vibration velocities increased by a factor of up to 1000during close-proximity helicopter flight as compared to ambient conditionsimmediately prior and that precise spectral alignment between infrasoundand landform natural frequencies is required to excite resonance. We defineadmittance as the ratio of vibration velocity to infrasound pressure andrecorded values of up to 0.11 mm s−1 Pa−1. While our resultsdemonstrate a strong vibration response, the measured velocities are lowerthan likely instantaneously damaging values. Our results serve as a basisfor predicting unfavorable degradation of culturally significant rocklandforms due to regular helicopter overflights.
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- Award ID(s):
- 1831283
- NSF-PAR ID:
- 10378594
- Date Published:
- Journal Name:
- Earth Surface Dynamics
- Volume:
- 9
- Issue:
- 6
- ISSN:
- 2196-632X
- Page Range / eLocation ID:
- 1459 to 1479
- 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/10.5194/esurf-9-1459-2021
