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 graduated geophysics student from the University of Utah Riley Finnegan measured the ambient vibrations of 14 large-scale structures in southern Utah, then included those measurements in a new dataset of 32 similar structures in Utah and beyond. The study was published in Seismological research letters.
“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,” said Devin McPhillipsa seismic geologist from the US Geological Survey 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 amazing structures.
“These data provide information for understanding how these landforms might react to blasting work done for road construction or other information for vibration damage risk or assessments,” Finnegan said.
Climbers help with data collection
Previous research has revealed 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 difficult.
“Individual data for each element can be incredibly difficult to obtain, usually involving technical escalation,” said Jeff Mooreco-author of the new study.
For the current study, the scientists teamed up with accomplished rock climbers led by co-authors Kathryn Vollinger and Jackson Bodtker, who followed established climbing routes up to 120 meters (~400 feet) high 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, noting fundamental frequencies of 0.8 to 15 hertz, or cycles per second, which were inversely proportional to tower size. They were unable to model four towers due to the camera’s exposure to sunlight and hard-to-measure rotational movements, such as twisting.
“You can kind of think of a tower like a guitar string that’s turned sideways,” Finnegan said. “You play guitar string, and it vibrates and resonates at certain frequencies, and we hear those frequencies.” Likewise, the towers vibrate at certain frequencies, although they cannot be heard on the ground. The researchers created amplified audio recordings towers with their 3D models.
Predictive modeling with seismic potential
In addition to measuring frequencies, mode shapes, and damping rates (a measure of the decrease in rocking motion over time) in the 14 structures, Finnegan and his team collected frequency data and heights. from previous studies and consulting reports for structures elsewhere in Utah, as well as in Arizona, France and Israel. By combining the compiled data with their own measurements, the researchers confirmed a formula for determining the fundamental frequency of a structure based on 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 difficult climbs; 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 recorded megathrust 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 much shaking they can withstand, we can assign a maximum value to the strength of past shaking, which is very useful for refining risk models. seismic.”
—Robin Donovan (@RobinKD), science writer