Watch these towers get wiggly

The mission of the National Park Service has always been to set aside and preserve land for the enjoyment of future generations. But as parks spring up around areas of natural splendor or cultural significance, they bring with them an unseen risk: vibrational energy. 

In addition to natural vibrations caused by storms, wind or earthquakes, our cars, trains, aircraft and even feet collectively create an invisible yet chaotic soundscape. In the American Southwest — an area known for its bizarre geological formations and extensive Indigenous ruins — these vibrations can threaten the integrity of such structures, potentially leading to their collapse. 

Jeffrey Moore, a geological engineer at the University of Utah, has spent more than a decade assessing the risk that vibrational energy poses to desert towers and arches, along with structures built over centuries by Native peoples. His work has taken him all across the Southwest, from Canyonlands and Arches national parks in southeast Utah to Colorado’s Rocky Mountain National Park, where he helps land managers develop policies that minimize the risk sound poses to sensitive sites. 

The challenge, Moore says, is that our soundscape has gotten increasingly loud over time. Many of the structures he studies formed over millennia or were built hundreds of years ago, and while they were always experiencing natural vibrations, we’ve now added a lot more noise to the mix. “If you look at a place like Arches National Park, we built a railroad just outside the park boundary, and you can detect those vibrations in arches that are miles away,” he says, adding that aviation tourism has also emerged as a significant source of sound. “Humans are generating energy in the environment that just wasn’t there over the last thousands of years.”

To study these vibrations, Moore and his team begin by creating intricate, high-resolution models of each structure using a drone (Moore says the frequencies that drones create are minimally invasive). They then deploy seismometers, the same equipment we use to detect earthquakes, sometimes by having rock climbers haul them up and then paraglide off. These machines pick up vibrations at a level that humans cannot necessarily hear or feel, typically under 20 hertz, but it’s these frequencies that pose the most risk to delicate formations. 

Each structure, be it an arch or a tower, has its own natural sway, such that you can identify it just from sound. Moore previously found that Castleton Tower, a 400-foot sandstone column outside Moab, vibrates at about the same frequency as the human heart, for example. And like a doctor listening to a heartbeat, scientists can learn something from listening to these sounds. 

When vibrations that match that same frequency strike a structure, they resonate and amplify, making the sway even more pronounced and potentially causing cracks. Every few years, memorable structures crumble to the ground, including Arches National Park’s Wall Arch and Rainbow Arch in 2008 and 2018, respectively, as well as Double Arch in Glen Canyon National Recreation Area last year. In the latter, officials cited “frequent foot traffic” and “erosion from waves” as possible explanations for the feature’s demise.

“These features have a life span that can be influenced or damaged by manmade interventions,” Michelle Kerns, superintendent of the recreation area that spans the border of Utah and Arizona, said at the time.

To date, Moore has studied vibrational damage in dozens of arches and towers, often with the goal of informing decisions around their management. In 2016, Moore and his partners launched a study at Rainbow Bridge National Monument near Lake Powell to determine whether human-caused vibrations might be posing a risk to the area’s most iconic structure. At 290 feet high and 275 feet wide, Rainbow Bridge is one of the largest natural bridges in the world and is also a sacred site for several Native American nations, including the Navajo, Hopi, San Juan, Southern Paiute, Kaibab Paiute and White Mesa Ute.

The team found that Rainbow Bridge vibrates in eight distinct directions, including up and down, side to side, and in a twisting motion. The four seismometers detected vibrations caused by waves in Lake Powell, as well as the vibrations caused by a human-made earthquake in Oklahoma, more than 1,000 miles away. Collectively, the results suggest that human disturbances could ultimately damage the structure without careful management. While the researchers didn’t detect any overt damage, such as cracks, the resulting small vibrations from wave activity at Lake Powell kept the overall vibration of Rainbow Bridge at a higher level than it would have been had the lake not been there. The effects of vibration are cumulative over time, Moore says, and can affect the structures’ overall longevity. 

In the years since, Moore has focused more specifically on the risks that helicopters pose. These aircraft, he says, generate the lowest frequencies, particularly when they fly at something head-on. Both commercial tour companies and private pilots have previously been documented flying too close to protected sites, particularly during COVID-19, when many parks were shut down. Moore says that he heard anecdotally about private flights buzzing low through Navajo National Monument and that “a yahoo” even landed a helicopter on Pritchett Arch outside Moab. “It really does make me angry to think about,” he says.

In 2023, Moore’s work contributed to a collective agreement between the Park Service and the Federal Aviation Administration setting out guidelines for tour companies operating over Glen Canyon National Recreation Area and Rainbow Bridge National Recreation Area. Today, all nine groups have voluntarily signed on, meaning they all operate under a shared understanding about where and how they can fly. “It’s always nice to see your science go towards establishing solid policies,” Moore says. “The goal is ultimately not to prevent helicopter tourism in its entirety; it’s to develop safe practices that balance these different needs.”

Today, Moore is similarly working with land managers at Tonto and Montezuma Castle national monuments in Arizona, as well as at Hovenweep National Monument in Utah, to study the risks vibrations could pose to Indigenous ruins. At these sites, visitors can tour structures that have stood for hundreds of years. That they’ve lasted for centuries is a testament to their builders, but aviation tourism poses a new and serious risk to their longevity. In 2020, a tour company was fined for flying a helicopter nearly level with Montezuma Castle, coming within 100 feet of the cliff dwellings there.

In recognition of how sensitive the structures are, Moore has changed up his methods, deploying a new type of crowdsourced seismometer that is much smaller and therefore less likely to leave any lasting impressions on the site. While the work has not yet been published, Moore says the team has captured examples of both natural and human-made vibrations, although there’s no evidence yet that they’ve contributed to any damage.

Matt Guebard, an integrated resource manager in the Park Service’s southern Arizona office, will use Moore’s data to inform future plans. “Working with Dr. Moore and the University of Utah has allowed us to utilize specialized expertise that we don’t have access to in our small parks,” he says, adding that “we hope to use the results of the study to better articulate the types of scenarios where damage is more likely and ultimately work closely and cooperatively with the aviation community to avoid them.”