February 08 Issue - Employee Monthly Magazine

Ever wonder what success sounds like?

Earthquake-triggering hunch pays off

Laboratory researcher Paul Johnson holds a block of acrylic plastic used in studying cracked solids by observing the interaction dynamics of elastic waves within solids. Ultimately this may offer more clues to understanding earthquake phenomenon. Photo by Dixon Wolf

For Laboratory researcher Paul Johnson of Geophysics (EES-11), the "eureka moment" sounded like a whip crack. It was a sound that some of his research colleagues didn't expect to hear.

Johnson had been following research by Joan Gomberg of the U.S. Geological Survey indicating that earthquake activity sometimes increases thousands of miles away from an earthquake epicenter, presumably out of range of the "aftershock zone" common for many quakes. He and several colleagues wondered how sound waves could trigger faraway quakes. His initial work on the topic with Gomberg and Xiaoping Jia of the Laboratoire de Physique des Matériaux Divisés et des Interfaces, Université de Marne-la-Vallés, appeared in Nature in 2005. In that work, they proposed how triggering could happen, supported by earthquake triggering observations.

More recently, Johnson contacted Chris Marone of Pennsylvania State University, who had developed a machine to mimic earthquake behavior. The machine squeezes plates atop a layer of tiny glass beads. The plates are similar to tectonic plates that surf above Earth's mantle. Like tectonic plates, when enough stress is applied to the plates in the earthquake machine, they skitter across the glass beads—unleashing an earthquake that can be observed in a laboratory setting.

"I told Chris I wanted to test whether sound waves could trigger an earthquake," said Johnson. "He was doubtful at first but welcomed a try." Johnson gathered his acoustic equipment and traveled to Penn State to modify the earthquake machine. To Marone's surprise and Johnson's surprise and satisfaction, sound waves applied for a short period just before an expected earthquake could induce smaller quakes.

"When the machine goes off and you get a quake, you get this really loud, sharp crack," said Johnson. "We found that sound could cause a quake immediately. That was truly a eureka moment. The most intriguing aspect, however, was that the applied sound was ‘remembered' by the glass beads, as manifested by delayed quakes, as well as delays in the occurrence of a larger, expected quake. We still are trying to understand how the memory is stored."

Johnson, Gomberg, and Marone joined Heather Savage and Matt Knuth of Penn State in publishing their findings in a recent issue of Nature. The research ultimately may help explain the mysterious behavior of earthquakes worldwide.

-James E. Rickman