Measuring the tempo of Utah’s Red Rock Tower

A rock tower called a “bike seat”. Credit: Geohazards Research Group

No matter how careful you look, you won’t see them moving.

You can’t hear their vibrations when you press your ears against the cold sandstone.

But a new study is that red rock The towers found throughout southern Utah and the Colorado Plateau are constantly moving, oscillating in a unique rhythm as unique as the dramatic profile for the depths of the blue desert sky.

Researchers at the University of Utah are familiar with how rock towers and arches sway, twist, and sway in response to distant earthquakes, winds, and even ocean waves. Their latest study resonates with the dynamic properties, that is, the frequency at which the rock vibrates and the way the rock deforms during that vibration, and how the beam is constructed.

Knowing these characteristics is important for understanding the seismic stability of rock towers and their susceptibility to dangerous vibrations. However, it is difficult to get the data you need because you have to travel dangerous terrain to reach the base of the tower. Then someone needs to climb them and place the seismograph on top.

However, with the help of experienced mountaineers, researchers at the University of Utah have measured the dynamic properties of 14 rock towers and fins in Utah and have unique data with varying heights and tower shapes. I created a set.

“This ability to predict the fundamental frequency of a tower using only the width, height and material properties of the tower is powerful. It does not require you to climb a 300 ft (100 m) tower using a seismograph. That’s why, “said Riley Finnegan, lead author and PhD student in geophysics. “And knowing this information is important for assessments related to tower seismic stability or potential vibration damage.”

The study is published at Seismology Research Letter Funded by the National Science Foundation and the University of Utah Undergraduate Lab.

One exaggerated animation of the bike seat movement mode. Credit: Geohazards Research Group

Scale the top of the tower

Finnegan, Jeff Moore, an associate professor of geology and geophysics, and colleagues have spent years measuring and cataloging the shapes of arches and other rocks to understand how they work. I did.

To get the seismographs to the top of these towers, researchers teamed up with mountaineering expert Kathryn Bollinger to climb the towers with her partner and carry the instruments to the top. Then they waited while the equipment was recording the data and brought them back.

The researchers also helped others. Jackson Botker, now a graduate of the University of Calgary, climbed three towers a day. Alex Dzubay, majoring in geophysics, scrambled a 1000-foot cliff to access one tower. Moore’s family also contributed to measure the dimensions of the rock tower in Arizona.

“There are so many talented, enthusiastic and kind people involved in the fieldwork,” says Finnegan. “After Kathryn climbed, our group went to three sites to fly a drone to create a 3D model. I personally could hardly reach the base of one of the towers, Even better, I started thinking about carrying equipment to the base. I’ll pull everything up. “

Respect the land

Some of the sites surveyed have special implications for local Native American tribes, such as the Valley of the Gods Tower in Bears Years, Utah. According to the Bears Ears Inter-Tribal Coalition, the valley is “… considered sacred to the Navajos. The Navajos interpret the giant sandstone monuments as ancient Navajo warriors frozen in stone. And there is time. “

One of the modes of Eagle Plume Tower, Utah, or an exaggerated animation of the main direction of travel. Credit: Geohazards Research Group

According to Finnegan, researchers met a White Horse High teacher at Montez McLeak near the Valley of the Gods, and one of the students, Weston Manygoat, joined them for fieldwork. “He was very cheerful and diligent, and we were very grateful for his help,” says Finnegan.

The research team sought to practice respectful visits during fieldwork. Others visiting the Bears Ears area are advised to visit the Friends of Cedar Mesa Visitor Center in Bluff, Utah, with reference to Union guidelines. Visitors need to stay on the marked trails, visit cultural places with respect, leave behind the cultural objects found, and avoid touching rock art.

“By recognizing that these towers are constantly moving, quivering, swaying, and quivering in response to the wind and energy flowing through the earth, visitors to these sometimes sacred landscapes add a layer of respect. And in the end we hope our measurements are inspiring, “the spirit of care for these amazingly unique places,” says Moore.

Rocks swaying like trees

In total, this study summarized the ambient vibration data of 14 rock towers and fins collected over several years. The team previously reported measurements from a single terrain, 120 m high Castleton Tower, but the new compilation is larger, wider and more versatile than the previously published dataset. It straddles the height and shape of the tower.

As a result, it was found that the fundamental frequency of the rock tower fluctuates between about 1 Hz (1 cycle per second) and 15 Hz, and the larger the tower, the lower the fundamental frequency. In general, towers bend and sway like trees and skyscrapers. At higher frequencies, the tower twists around the central axis.

“Perhaps the most amazing thing for me was how well our data was in line with theory, and how well our model supported the data,” says Finnegan. The theory predicts that the fundamental frequency at which the beam vibrates is proportional to the width of the beam divided by the square of the height. The Rock Tower mainly followed that relationship.

The predicted frequency of rock tower vibration was about 4% different from the observed data. And the predicted angle of tower movement was on average 14 ° off from the actual data. “

“Maybe I’m overly excited and surprised by this,” says Finnegan. You can now predict tower mode given geometry. “

The new measurements, along with previously published data, are the values ​​needed to assess seismic stability and vibration risk in various settings around the world for other rock towers, pillars and fins. Provides guidance for estimating the natural frequency to assess the potential strength of past tremors. According to Finnegan, knowing how to predict the properties of a rock tower makes it much easier to assess the condition of the tower with fewer measurements.

Some of the most rewarding times I’ve spent in the field are when I sit back and listen and imagine these towers moving, “says Moore. But it’s very realistic and it’s always (and always) happening. For me, this new perspective creates a new intimate connection with the landscape. ”

Utah Red Rock Metronome: Seismic Measurements Reveal Invisible Vibrations of Castleton Tower

For more information:
Riley Finnegan et al, Circumferential vibration modal analysis of natural rock towers and fins, Seismology Research Letter (2022). DOI: 10.1785 / 0220210325

Provided by
University of Utah

Quote: Https: // of Utah’s Red Rock Tower (February 16, 2022) acquired on February 16, 2022. Tempo measurement

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Measuring the tempo of Utah’s Red Rock Tower

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