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Rock Music in Utah – Eos

It’s easy to think that Utah’s sculptural red rock towers are immobile, if not immovable. Yet the rock towers twist, sway and sway imperceptibly in response to vibrations and seismic activity.

Recently, a geophysics graduate student at the University of Utah Riley Finnegan measured the ambient vibrations of 14 large-scale structures in southern Utah and then included those measurements in a new dataset of 32 similar structures in Utah and beyond. the study was published in Seismological investigation maps.

“These data provide information for understanding how these landforms might react to blasting work carried out to build roads or other entrances for vibration damage or risk assessments. »

” What [the researchers have] really done is to help us have confidence in our predictions about the specific frequencies at which these rocky towers will resonate,” he said. Devin McPhillips, a US Geological Survey seismic geologist who was not involved in the study.

In addition to vibrational risk assessment, the rock towers hold spiritual and cultural significance to the original occupants of these lands, including members of the Eastern Shoshone, Hopi, Navajo, Southern Paiute, Ute, and Zune tribes. . “These are culturally valuable landforms,” ​​Finnegan said. She hopes the data will be used not only to predict the impacts of natural disasters and man-made vibrations, but also to preserve the impressive structures.

“These data provide information to understand how these landforms might react to blasting work done to build roads or other inputs to assess vibration damage or risk,” Finnegan said.

Climbers help with data collection

Previous research has uncovered the impact of vibrations from helicopter flights on structures in Utah, and similar studies have measured the natural frequency of mountains. These measurements inform seismic risk assessments as well as risks from other types of vibration. But collecting metrics is a challenge.

“Individual data for each feature can be incredibly difficult to obtain, often involving technical escalation,” he said. Jeff Mooreco-author of the new study.

Climbers descend from Eagle Plume Tower in Utah after placing seismometers on the rock formation. Credit: Eric Albright

For the current study, the scientists teamed up with accomplished climbers led by co-authors Kathryn Vollinger and Jackson Bodtker, who followed established climbing routes up to 120 meters (~400 feet) in height to place seismometers, which work like sophisticated accelerometers, on top of rock formations. Using photographs, drone imagery and seismometer data, the researchers created 3D models of 10 of the 14 rock formations they attempted, examining fundamental frequencies from 0.8 to 15 hertz, or cycles per second, which were inversely proportional to formation size. .round. They were unable to model four towers due to camera exposure to sunlight and hard-to-measure rotational motions such as twisting.

“You can think of a tower like a guitar string that’s flipped over,” Finnegan said. “You pluck the guitar string, and it vibrates and resonates at certain frequencies, and we listen to those frequencies. Likewise, the towers vibrate at certain frequencies, although they are not audible on the ground. The researchers created amplified audio recordings of the towers with their 3D models.

This exaggerated model shows the predominant modes, or directions of motion, of the Eagle Plume Tower. Credit: Geological Hazards Research Group, University of Utah

Predictive modeling with seismic potential

In addition to measuring frequencies, mode shapes, and damping rates (a measure of the decrease in oscillating motion over time) on the 14 structures, Finnegan and his team collected frequency data and heights of round from previous studies and consulting reports for structures elsewhere in Utah. . as well as in Arizona, France and Israel. By combining the collected data with their own measurements, the researchers confirmed a formula for determining the fundamental frequency of a structure given its width, height and composition. Using this relationship, the researchers found they could roughly predict the natural frequency of unknown sandstone or conglomerate structures.

The researchers hope this confirmation will help others predict the dynamic and resonance properties of other rock towers without scaling issues; the calculation only requires width and height measurements that can be taken from the ground.

“If we know something about the composition of the feature and we know its shape, we can make a pretty good estimate of what the fundamental frequency will be,” McPhillips said. And these seemingly basic measures can have far-reaching implications, from refining earthquake hazard models to developing building codes in regions prone to seismic activity, like the Pacific Northwest. Such predictability becomes even more important when scientists like McPhillips are tasked with predicting outcomes in regions that have never experienced mega-earthquakes.

“Extrapolating from the limited historical data we have is potentially dangerous,” McPhillips said. Thus, data like Finnegan’s provides additional certainty and predictability. “If we know the age of these rock towers and can estimate how many shakings they can withstand, we can assign a maximum value to the strength of past shaking, and that’s really helpful for refining seismic hazard models.” . »

—Robin Donovan@RobinKD), science writer

Quote: Donovan, R. (2022), Rock Music in Utah, eos, 103, Posted May 3, 2022.
Text © 2022. The authors. CC BY-NC-ND 3.0
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