Ronald J. Scott

Here’s how an iconic landform near Moab sways like a skyscraper from natural earth movements

By Carter Williams, | Posted - Aug. 29, 2019 at 7:32 p.m.

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MOAB — Castleton Tower has been one of Grand County’s premier rock climbing sites since it was first climbed in 1961 — but have you ever heard how it sounds when it moves?

Yes, even a near 400-foot Wingate Sandstone spire makes a sound almost like a baritone metronome, and University of Utah researchers say that’s from wind and deep vibrations in the earth. It’s important, they say, because hearing rocks and measuring their natural movements can help us understand how natural and human-made vibrations can affect rock formations.

A new study published in the Bulletin of the Seismological Society of America on Tuesday, found that the Castleton Tower had resonance modes of 0.8 and 1.0 hertz in a natural setting emanating from energy from the earth's surface and wind — essentially meaning the tower sways about once per second, much like large buildings sway.

“Castleton Tower’s really dramatic appearance is really not just a static landform. It’s constantly moving,” said Jeffrey Moore, associate professor of geology and physics at the U. and corresponding author of the study. “In fact, every second of every day, it’s always in motion.”

Of course, that’s not necessarily new. Moore explained it’s well known that everything on earth is vibrating all the time. Riley Finnegan, a graduate student and co-author of the study, likened the movement to guitar strings.

“The thick ones have lower pitches, and the thin ones have higher pitches,” she explained in a news release.

Engineers have figured out how to design buildings that can react to movements from the wind and the ground, Moore said. However, his team has sought how natural structures adapt to similar conditions.

“Really, we know nothing about the properties, exactly, to predict how (a natural structure) will respond to ... let’s say an earthquake,” he said. “The background of this study is to record some of these essential properties of this really iconic landform in order to allow us to be able to conduct these sorts of analysis.”

Researchers were able to record the Castleton Tower data by enlisting the help of a pair of professional climbers who summited the rock formation and placed a seismometer at the base of the tower. It remained there for three hours to collect movement and movement sound.

The data gave the researchers a baseline idea of how much Castleton Tower moves on a daily basis. With it, Moore said the researchers were able to determine what they could expect from something like an earthquake by plugging in variables.

“What kind of earthquake might stimulate resonance in the tower? What kind of amplification can we exact at the top of the tower compared to the bottom of the tower? What kind of damping — that’s how quickly is the energy damped out of the system. All these things we were able to retrieve from our study,” he said.

It also allows researchers the ability to track natural formations over time. As Moore pointed out, Castleton Tower and other desert landforms were created from erosion over a long period of time. Weather and earth movement both factored into it. Adding in human interaction now, there are plenty of variables that can alter desert formations in the future.

“We want to know what the magnitude of those sources are,” he added. “We want to understand how these structures respond to those forces. Secondly, we want to understand: Could human sources stimulate the vibration of these landforms that might ultimately speed up the process of erosion?”

If researchers spot any changes in the data, then they can determine there has been some form of damage to the formation.

The research team has also recorded data from 10 other natural towers and 20 arches in Utah. Castleton Tower is among the largest of the rock towers they’ve studied. They plan to measure more natural features in the future.

“It’s the smaller towers and arches, especially, that might be susceptible to human vibration generation,” Moore said, “whereas the large rock formations like Castleton Tower appear that they should be less sensitive to those kinds of vibration sources."

You can hear the “voices” of the other rock features studied here.


Carter Williams

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