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.