Scientists listen to the singing of Utah's Rainbow Bridge

"Human activity has altered the earth's vibrational wavefield," said researcher Jeff Moore.
By Brooks Hays  |  Sept. 21, 2016 at 4:20 PM
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SALT LAKE CITY, Sept. 21 (UPI) -- Scientists at the University of Utah have recorded all the different frequencies at which the Rainbow Bridge resonates.

The new research, published this week in the journal Geophysical Research Letters, pinpoints the many natural and man-made sources of the bridge's humming.

A team of researchers installed a series of seismic sensors in and around the Rainbow Bridge to monitor the rock formations' various modes of vibration.

"A mode is the pattern of vibrational motion at a given frequency," geophysics professor Jeff Moore explained in a news release. "Think of a guitar string. When you pluck a guitar string you generate one main tone plus several overtones on top of that. Those are all different modes."

Moore and his research partners were keen on understanding what vibrational frequencies cause the Rainbow Bridge to resonate. Every object, given its structure and composition, prefers to vibrate at certain frequencies. Objects can even naturally amplify their resonant frequencies, sometimes inducing self-destruction.

Over two days of observations, scientists and their sensors identified eight main modes of vibration -- some vertically oriented, some horizontally oriented and some twisting.

Though most of the time the bridge simply quivered at a low hum, several man-made and natural sources induce more vigorous vibrations.

Mode 1, which features vibrations at a frequency of approximately 1.1 Hertz, corresponds with the waves in nearby Lake Powell. The bridge also resonated at the frequencies of local earthquakes.

"Many things we do are actually felt by Rainbow Bridge, which is extremely remote," Moore said. "Human activity has altered the earth's vibrational wavefield."

One thing researchers aren't sure about yet, is how man-made vibrations alter the structural integrity of vulnerable rock formations like Rainbow Bridge.

In addition to listening to the bridge, researchers also imaged it, using a series of photographs to model it in 3D. Researchers hope their new model, as well as the baseline of resonant frequencies, will allow them to monitor the rock's structural changes over time.

"We hope to provide a new way for people to look at the bridge as a dynamic, lively feature that's constantly vibrating and constantly moving," Moore concluded. "You get a new understanding of the bridge as a living feature rather than a static structure."

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