When bridges and buildings begin to vibrate, whether from the wind or traffic or another stressor, they can literally shake themselves to pieces. Watch a 1940 clip of the Tacoma Narrows Bridge galloping up and down and then ripping like soggy cardboard to get a sense of the effects. This only happens when the vibrations happen to match what's called the resonant frequency of the structure, and engineers try to make sure this won't happen. Skyscrapers even have devices called dampers on their roofs to absorb energy.
But natural, geological bridges like the great sandstone arches of Arches National Park in Utah, have no such defenses; they are still standing because over the eons they have shed pieces of themselves and adjusted their tension to weather the energy that washes up against them. However, with humans around—along with our helicopters, boats, highways, and everything else—the landscape of that energy has changed.
Jeffrey Moore, a professor of geology at University of Utah, is looking to find out what the resonant frequencies of natural arches are and what vibrations, exactly, they are vulnerable to. In a recent study of the Rainbow Bridge in the remote Four Corners Region of the Southwest, published in Geophysical Research Letters, he and his team report something stunning: The bridge is so sensitive it picked up what was likely a man-made earthquake in Oklahoma, hundreds of miles away. The waves of nearby Lake Powell showed up in its tremblings, too. The impression the report conveys is of a structure of tremendous sensitivity and resilience that should be monitored going forward to see how it responds to shocks.
Moore and his group did their study at the request of the National Park Service, prompted in its turn by a committee of Native American tribal organizations, for whom the bridge is a sacred site. Last March they brought four seismometers, which measure vibration, to the bridge. A researcher rappelling down onto the span from a cliff placed two on the structure itself, and the others were deployed on either side of the bridge, some meters away. The researchers collected readings for 22 hours, and build a mathematical model of the structure using the measurements they gathered, and information about its shape.
Right away the data showed that the arch was picking up a range of vibrations. In the juddering lines of the seismograph readings they could watch the winds along the canyon dying down over the day, as well as the reverberations of the waves from Lake Powell. And “we could see things in our data straight away that looked like earthquakes,” says Moore. Two small local earthquakes rippled through during the measurement period, but there was one set of vibrations that looked different, with a much narrower range of energy that suggested it had traveled a long way. This appears to be from an earthquake induced by drilling to store waste water from oil production on the Oklahoma-Kansas border, Moore says. “The most interesting thing is just the recognition that this distant event is felt, is absorbed by Rainbow Bridge ... Even here in this incredible remote place, this earthquake from Oklahoma is rattling the bridge.”
The distant earthquake and the vibrations from Lake Powell, which is an artificial reservoir, activated the bridge at one of its resonant frequencies, the group notes. It's not clear what that means for its long-term stability. But it is a reminder that our activities are not anywhere near as constrained in space as we like to think they are. The model the team made provides a benchmark for the bridge going forward, however—future measurements will reveal whether the bridge's properties are changing as a result of vibrational damage. “We're poised to come back and re-measure the resonant properties,” says Moore. If a structure develops cracks and grows softer and less stiff, its resonant frequencies will drop. “If we come back in five years and all the resonant frequencies have dropped by, say, 10 percent, then we'd have some data to suggest that the arch has been damaged during that time,” he says. Moore hopes that this information, which his group is gathering about numerous natural bridges in the Southwest, will help the Park Service and other management organizations make decisions that will help extend the lifetimes of these magnificent structures.