Scientists call the tendency of physical objects to vibrate when excited by a certain frequency “resonance.” A guitar string, for example, oscillates in response to tones sounded in the same room. A visually memorable example of resonance is the 1940 collapse of the original Tacoma Narrows Bridge. Aptly nicknamed “Galloping Gertie,” the Puget Sound suspension span twisted and failed due to wind-induced vibrations.
Utah State University geophysicist Tony Lowry suggests that movements observed at regular intervals on the earth’s deep tectonic faults are resonant responses to the weight of groundwater and ocean water shifted about by weather cycles. His research, funded by the National Science Foundation, is described in the Aug. 17 issue of Nature.
“Fault movements similar to earthquakes, but much slower, have been recorded at various subduction zones around the world, including southern Mexico, Japan, New Zealand and the United States’ Pacific Northwest,” says Lowry, who recently joined USU’s Geology Department as an assistant professor. “But the underlying causes of these events have been poorly understood.”
The movements, known as “slow slip events” or “silent earthquakes,” are actually not earthquakes and produce no noticeable ground shaking, he said. And unlike earthquakes, which recur at unpredictable times, slow slip events typically occur at regular intervals of six to 18 months.
While researching slow slip phenomena in southern Mexico, Lowry found that events occurred at almost exactly the same time each year. Other researchers had already noted that repeating slip in the Pacific Northwest closely matched the frequency of the “Chandler wobble,” a small shift in Earth rotation caused by changes in the weight of ocean basins. “This suggested to me that the slow slip events might have something to do with the changes in pressure caused by annual and other cycles of surface fluid movements,” he says.
Weather cycles move a lot of mass around the Earth’s surface and changes in atmospheric pressure also impact rock stress at depth. Though tiny, relative to tectonic stress, these changes are large enough to excite fault movement at their resonant frequencies. “Fault slip resonance with climatic mass cycles explains why slip events are periodic, and the dependence of resonant frequency on fault properties explains why slip periods differ from place to place,” says Lowry.
Understanding the connection between surface weather and fault movement provides a potentially valuable tool for probing faults and better understanding their behavior, he says. “This knowledge will help to illuminate the frictional properties of faults, which should improve our understanding of earthquakes.”