7 August 2015
By Larry O’Hanlon
For the first time, scientists have found a way to detect if the breathtaking natural arches of Utah’s Canyonlands and Arches national parks are suffering from internal damage that could lead to their collapse.
A new study employed seismometers to listen to the arches’ natural humming and monitor them for any ominous changes. The study’s authors tested the new technique on the iconic 27-meter (88-foot) long Mesa Arch. They found that the arch is not currently suffering from progressing damage, but it does vibrate in tune with changes in temperature.
“These arches are not only sitting there humming away, they are also moving up and down,” said Jeff Moore of the University of Utah in Salt Lake City. “We have a good handle on the vibration dynamics of Mesa Arch right now. We’re now set to study it into the future and monitor for non-reversible changes.”
Canyonlands and Arches national parks in southern Utah are home to more than 2,000 natural arches. These majestic sandstone arches have stood for thousands of years, but they do periodically collapse, threatening the safety of the hundreds of thousands of people that visit the parks every year. For example, in August 2008, Wall Arch, which spanned 22 meters (71 feet) collapsed unexpectedly along a popular trail. A portion of the same trail was closed when Landscape Arch, the longest natural arch in North America at 88 meters (290 feet), lost portions of rock in 1991 and 1995. In all, at least 43 arches have collapsed since 1977, according to rangers at Arches National Park. . A few of these were destroyed by vandals, but most were natural collapses caused by erosion.
The need for a method to monitor the health of arches is clear, given the failure rate and cultural significance of natural arches in the region, said Moore.
“What we hear all the time when we’re out there is, ‘Is the arch going to fall down?,’” said Moore, a co-author of the new study accepted for publication in Geophysical Research Letters, a journal of the American Geophysical Union. “Finding internal damage is very difficult. There hasn’t been a good way without slapping instruments all over the rock.” And that solution is likely not something the arch-admiring and picture-taking public is likely to accept, he added.
So Moore’s team took an approach used to monitor the health of buildings, bridges and other engineered structures. They employed a pair of small, portable, hand-held seismometers that they placed on and near the arches for one-hour increments. Through these instruments they discovered the various subtle ways that the arches imperceptibly twist, turn, nod and jiggle – analogous to the different ways a guitar string vibrates when plucked.
The arches are being “plucked” by the wind and the constant vibration of the Earth, Moore said. These various kinds of movement, or modes of vibration, each have their own resonant frequency, or tone that is recorded by the seismometer. Once the researchers identify these tones for a particular arch, they can periodically return to see if the arch’s tune has changed — a sign of internal changes that indicate damage, Moore said.
“If the resonance properties change, there is some change within the structure,” said Moore. “We show that yes, this method works. The arches are really good resonators. So we can measure them and track them over time as a measure of structural change.”
The new study describes the use of the technique at Mesa Arch in Canyonlands National Park. Mesa Arch is about 2.5 meters (8 feet) thick, 3 meters (10 feet) wide and spans 27 meters (88 feet), and is one of the most photographed arches in the world.
The study’s authors found that Mesa Arch has four primary modes of vibration which are, loosely speaking, bending sideways, jiggling in an s-shaped sideways motion, moving up and down, and a wave-like movement along the length of the arch.
The good news is that in the nine times the team checked Mesa Arch’s tune over a year-and-a-half they did not detect any permanent changes that would indicate damage, Moore said.
“We didn’t see any permanent change, but did see transient changes,” Moore said. To discover the source of the short-lived, transient changes, they conducted an experiment in which they monitored Mesa Arch continuously for two-and-a-half days rather than just the one hour they usually monitor the arch.
The team saw that the arch experienced cyclical changes that coincided with changes in the temperature of the rock surface. As the arch heats up in the sun, the rocks that make up the arch expand, causing the entire arch to swell. Stresses from this expansion cause the arch to subtly bow upwards, stiffening the rock and raising the resonant frequency – or tone – of the arch. The reverse happens when the rocks cool: the arch sags downward and the resonant frequency goes down. Moore said the team thinks these transient changes also occur on seasonal cycles.
The researchers have also begun to study other arches in the area. The number of significant modes an arch has depends on the structure of the particular arch, Moore said.
The new technique also could be applied to other rock structures, like coastal sea mounts, natural bridges or pinnacles, Moore said. For that reason Moore and his colleagues are giving away all our data generated in this project freely for unrestricted use via http://geohazards.earth.utah.edu/data.html.
“There’s no reason this couldn’t be applied to all bedrock structures,” he said.
– Freelance science journalist Larry O’Hanlon acts as AGU’s blogs manager and social media coordinator.