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It starts out as a rumble, like something heard underwater. It rises and falls in waves that pass one after another. But it couldn't be farther from the sea. This is what the red rock arches of the Colorado Plateau sound like. Seismic vibrations that are almost imperceptible to human ears ripple through the rocks.
Researchers use recordings of the vibrations to study the arches' structural stability. Scientists characterize how waves resonate throughout the bridge, causing small movements that, when exaggerated, look like wobbles on a plate of gelatin. People even affect the arches' stability: Researchers found that a helicopter flying close to an arch in Utah's Bryce Canyon caused the arch to vibrate with an amplitude one hundred times greater than normal.Image credit: Alison Starr
