Scientists monitoring the vibrations of natural rock arches have found that the resonant frequencies of arches change throughout the day, according to a press release from the Seismological Society of America.
There are thousands of arches in southern Utah. Researchers found that those arches vibrate slightly as they are shaken by wind, earthquakes and human disturbances.
University of Utah doctoral student Paul Geimer led the study. He cites an incident in Arches National Park as a motivation for his research.
“One of the main drivers of the research was the collapse of Wall Arch in 2008 in Arches National Park [in Utah], right along the trail,” Geimer said. “It happened in the middle of a calm night in summer with no indication that anything was about to happen.”
The press release states that Geimer’s advisor, University of Utah geoscientist Jeff Moore, had also received requests from the National Park Service and from Native American tribes worried about the future of Rainbow Bridge, near Lake Powell.
“They wanted to see if we could tell them more about the potential dangers from helicopters, and about the vibration levels that the bridge might be experiencing from these manmade sources,” Geimer said. He noted that monitoring the vibrations is a well-established method for detecting and measuring damage in man-made structures. He applied the same logic to natural rock arches.
Geimer and colleagues collected a year of continuous data on the vibrations of Aqueduct Arch in Utah. The unusually long recording time meant that the scientists were able to see how changes in vibrations correlated with changes in the environment. As a result, they were able to make predictive models for how environmental conditions affect the rock.
Resonant frequencies are highest when temperatures are below freezing or at their hottest in the summer because extreme temperatures cause rocks to stiffen, changing how the arches vibrate. While it is likely that the temperature changes weaken the arches over time, researchers were not able to find a frequency change that indicates an arch is about to collapse.
Meanwhile, Kate Cannon, superintendent of the Southeast Utah Group of National Parks, said that Arches National Park has its own monitoring systems.
“We have instruments in place to measure movements in the rocks in order to learn more about them,” Cannon said. “… Where there’s potential rock fall in the vicinity of busy areas … we record the changes because the rocks actually move, they expand and contract with temperature and so forth. We’re just keeping an eye on them to see if there begins to be any more significant movement that might be an early warning.”