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In seemingly silent immobility, Rainbow Bridge has stood for millennia arcing over a chasm connected to southern Utah's Glen Canyon.
But geologist Jeff Moore and University of Utah colleagues have listened to this iconic sandstone feature and found that it has a lot to say. Their sensitive equipment can hear it rattle and hum as it jostles imperceptibly up and down and side to side.
Vibrations constantly course through Rainbow Bridge, which resonates like a guitar string, but much of this lithic music doesn't come from natural sources.
Moore's research discovered that seismic energy generated by water lapping the shores of the nearby Lake Powell reservoir and an Oklahoma tremor triggered by oil and gas activity could be detected in the arch's vibrations. This research, part of a broader investigation of Utah's arches and bridges, may help determine whether these geological features are experiencing internal stresses that could lead to a premature collapse, said Moore, lead author on a study published Wednesday in the journal Geophysical Research Letters.
"Rainbow Bridge is an exceptionally rare and delicate landform, representing a balance between forces of erosion and resistance," said Moore, an assistant professor of geology. "The movements are small and the changes are slow, but just because we can't see it doesn't mean we can't hear it. Our research is [done] to help people see it in a new way. We are trying to get a sense of the balance between their strength and the forces being applied to them."
These geologists adapted equipment designed to perform non-invasive examinations of buildings' structural integrity in search of internal damage and stresses.
"We are merging structural engineering and geomechanics and seismology in this project," Moore said.
The goal is to characterize the stone features' "ambient resonance" and monitor it over time. Changes could reveal whether something is structurally amiss inside the rock.
Utah boasts the world's highest concentrations of natural bridges and arches, sculpted out of Entrada, Carmel and other sandstone formations by wind and rain. Arches National Park alone is home to 2,000.
"These things are not alive in a biological sense, but they have a life of their own," said Eric Bilderback, a geomorphologist with the National Park Services' Geologic Resources Division. "They are changing even when we look at them in our narrow human time frames. We have the technology to measure that and bring it to life. It has incredible potential as an outreach tool and to bring another degree of wonder to these features that are already wondrous."
Occasionally, these features change in sudden and catastrophic ways. That can pose a threat to public safety, especially in places like Arches' Windows, where thousands of park visitors linger under these vaulting forms on a daily basis.
In August 2008, the 71-foot-long Wall Arch collapsed just off Arches' popular Devil's Garden trail. In the 1990s, the nearby Landscape Arch twice lost big pieces, and park officials have permanently closed a trail under this 290-foot feature.
"We want to make sure people have access to their public lands but we want to do what we can to make them aware that there are risks associated with rock fall," Bilderback said.
Moore likes to record arches and bridges' vibrations over multiple visits, but the Rainbow Bridge study was a one-time shot. This is because the formation is hard to get to and it is revered by several American Indian tribes, whose approval was needed for the U. team to study the bridge.
It is visited by about 85,000 people every year. Most arrive by boat from Page, Ariz., and all are asked to not walk under it.
"It's a sacred place. Keeping it peaceful is important. In taking the measurements we need to be sensitive," Moore said.
The team visited the bridge, which spans Bridge Creek about a mile from Lake Powell, on March 24, 2015. They rigged two seismometers to the span itself, one underneath and a fourth on a connected knoll, then recorded vibrations for 20 straight hours. They detected eight distinct modes of vibrations ranging in frequency from 1 to 6 Hertz.
Moore's team also produced a 3-D model of the formation, among the world's largest natural spans at 270 feet long and 290 feet high, and determined it weighs 110,000 tons, comparable to a Nimitz-class aircraft carrier.