Anthropogenic Sources Stimulate Resonance of a Natural Rock Bridge


Rainbow Bridge in Rainbow Bridge National Monument, Utah. Photo courtesy of U.S.G.S.

by Denise Meeks

 Rainbow Bridge is a spectacular geologic wonder. And, like many of the bridges and arches in Utah, it has been singing for hundreds of millions of years. A unique collaboration among geologists, cultural anthropologists and members of Native American tribes gave it a voice that humans can now hear. And Rainbow Bridge has a lot to tell us.

This story began when members of the San Juan and Kaibab tribes, part of the Rainbow Bridge Native American Consultation Committee, contacted the National Park Service.  Concerned about the sound effects that tour helicopters might have on Rainbow Bridge and Utah’s other bridges and arches, tribe members asked the National Park Service to investigate. The task was given to Erik Stanfield, Cultural Resource Technician for the National Park Service at Glen Canyon National Recreation Area/Rainbow Bridge National Monument.

Stanfield, who has a sense of human relationships with the land, is completing a master’s degree in sociocultural anthropology at Northern Arizona University, where he was about to give a presentation on Rainbow Bridge.

Differences [in how one views the bridge] are based on cultural background. Some look at it scientifically, but local tribe visitors have a different relationship with the land,” Stanfield said. “It speaks to their history and ancestors. It brings out strong positive emotions.”

Stanfield became the advocate for the tribes. He explained that the National Park Service doesn’t have the expertise or resources to evaluate the physical health of Rainbow Bridge, so he contacted University of Utah Geology & Geophysics Associate Professor Jeff Moore, who holds a doctoral degree in civil engineering. Moore is a geologic hazards researcher who began his academic career at the University of Arizona where he earned a Bachelor of Science in geologic engineering. He is an advocate for the health of Utah’s natural structures.

“It’s natural sculptural perfection,” Moore said. “It’s special because it has a perfect, curving, bulky and smooth form. The scale and the sculpture are special. It also bears a resemblance to an inverted catenary arch.”

Moore and his team, including his geology doctoral student Riley Finnegan, were already performing seismic studies of 20 other arches and several towers in Utah, including Landscape Arch and Delicate Arch. Those studies have not yet been published.

Finnegan wanted to engage in research that had an impact on society and our relationship with conservation and the environment, so she joined Moore’s team.

“It’s an incredible landform, it looks perfect,” Finnegan said. “The form that makes it a strong bridge, shaped by nature, [but] society has advanced technologically, [and] we don’t know how technology will affect natural systems in the future and what our impact will be.”

Finnegan now studies anthropogenic effects, vibrations induced by human behavior, technology and man-made structures that affect arches and bridges. Her research analyzes the frequency of the sounds, both those we can and cannot hear, produced by helicopters. Finnegan determined that helicopters can generate very low frequency sounds, between 11 hertz (cycles per second) and 15 hertz that humans can’t hear. These frequencies may shake geological structures.

“[There are] forces that the bridge grew up with rather than what its dealing with now” Moore said. “We have to think about how the forces compare to the normal force that the bridge feels normally just under winds.”

In March 2015, to measure these anthropogenic forces, Moore and his team placed two sensors on the bridge and two nearby on the ground. For 22 hours these sensors listened as the bridge vibrated. The team detected three earthquakes. One was a human-induced earthquake near the Kansas-Oklahoma border.

“The energy that arrived from that earthquake was perfectly tuned to shake Rainbow Bridge at about 1 hertz,” Moore explained. “Rainbow Bridge was perfectly attuned to pick up those vibrations.”

That one event didn’t damage the bridge, but the possible cumulative effect of years of human activity is unknown.

Lake Powell might also damage the bridge. In 1980, Lake Powell, created by the Glen Canyon Dam, filled to its maximum elevation of 3,700 feet. The 40 years that have passed are the blink-of-an-eye compared with the bridge’s 200 million years of harmonious existence with wind and rain.

Moore explained that one of his colleagues analyzed data around lakes all over the world and found that lakes, including Lake Powell, naturally generate seismic energy at 1 hertz, the frequency of the waves moving across a lake surface due to wind. That 1 hertz frequency corresponds perfectly to the natural frequency of Rainbow Bridge, generating continuous, but very small vibrations in the bridge.

“The lake is shaking the bridge,” Moore said.

The team doesn’t have enough data yet to understand long-term effect of Lake Powell on the Bridge.

Rainbow Bridge is more than a fantastic geologic structure. Finnegan explained that the bridge is a sacred site for the tribes at the Four Corners area. Each of the tribes has its own stories and connections to the bridge.

“Anybody who visits Rainbow Bridge will recognize its uniqueness. It provides an inspiring feeling to anyone who goes there,” Finnegan said. “It’s a window the next the world, a way to communicate with ancestors.”

The tribes’ concerns about helicopter noise was the result of lawsuits between the tribes and the federal government in the 1970s, 1980s and 1990s based on potential cultural property damage. One outcome of these suits was the National Parks Air Tour Management Act of 2000, requiring air tour operators flying over national parks or tribal lands to apply to the FAA for permission. The Act requires the FAA and the National Park Service to create air tour management plans for parks or tribal lands if tour operators submit applications.

The FAA Modernization and Reform Act of 2012 allowed the FAA and the National Park Service to enter into voluntary agreements with air tour operators instead of developing management plans. The 2012 amendment exempted national parks with 50 or fewer annual tours from the management plan and voluntary agreement requirements.

At Rainbow Bridge, the air tour operators and the National Park Service created voluntary agreements to “give the bridge a rest,” Moore explained.

The team has determined that helicopters are unlikely to damage the bridge because they don’t create the vibrations that are in tune with the bridge’s structure, but they don’t have enough long-term data to be sure.Even though air tours are regulated by the FAA, the National Park Service acted as an advocate for the tribes, who are concerned about preserving their cultural heritage. The National Park Service is also concerned with protection of national geologic treasures, and the FAA is concerned about safety.

“Everybody felt like their voices were heard,” Moore said.

Some members of the research team returned to Rainbow Bridge in 2017 and 2019 to remeasure the Bridge’s natural frequencies. Their data showed that the natural frequencies had not changed since they began their measurements in 2015. Moore said this suggests that there has been no permanent damage to the Bridge during that time, but that they don’t know the long-term effects.

Moore and Finnegan visit schools and give public lectures to help people connect to the Bridge and other natural wonders.

To help the public understand the importance of Rainbow Bridge and other natural bridges and towers, Moore and his colleagues converted the sounds made by the bridges into sounds that humans could hear. They increased the frequency by a factor of 25 and posted it on their public website. Moore played the sounds for 10,000 children at the 2018 STEM Expo. Now we can all hear the arches and bridges sing. We need to listen.