SALT LAKE CITY — Dozens of arches across Utah have collapsed in the last two decades, prompting new research that examines the potential long-term effects of helicopter tours and the vibrations they emit as they do their flybys over popular tourist attractions.
While the collapses of Rainbow Arch and Wall Arch at Arches National Park are not directly linked to vibrations from passing helicopters, the increase in the frequency of tours led the Native American Consultation Committee, which advises the National Park Service, to ask for a more in-depth look.
At the fall meeting of the American Geophysical Union in San Francisco, Riley Finnegan, of the University of Utah, reported that the low-frequency sound waves produced by helicopters can magnify the natural vibrations of arches 100-fold, potentially weakening the culturally valuable resources and shortening their lifespans.
The research was published this month in Physics and involved placing seismometers and geophones on top of five arches to record the velocity of oscillations, the maximum speed with which the surface moved when vibrating.
The team monitored between two and nine helicopter flybys for the different arches over the course of the study. For each flyby, the team measured intense bursts of infrasound, up to 100 decibels. This high-intensity, low-frequency would be inaudible to humans, but for the arches, Finnegan said, it would be akin to being at a live concert.
At Squint Arch, where the velocity of tremors was highest, the team measured vibrations with a velocity 100 times greater than the vibrations caused by the Earth’s natural background rumble. Finnegan said she thinks only earthquakes of magnitude 4 could cause vibrations like those induced by helicopters, and they occur “maybe once in a hundred years.”
Researchers don’t believe the vibrations are causing instantaneous damage, but prolonged exposure could lead to additional cracks in the geologic formations.
The potential impact of the aerial tours comes as most of the 85,000 annual visitors to Rainbow Bridge Natural Monument elect to see the 290-foot tall structure by boat or chopper.
It is accessible by foot, but only after a three or four day hike through difficult terrain.
“We have over 6,000 arches documented around (Utah) and an entire national park dedicated to how great they are. We want to make sure that we aren’t inadvertently affecting the health of these really precious features,” Finnegan said.