Researchers explore quakes' shaky memories

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By Roger Snodgrass

A magnitude 7.4 earthquake centered in Yucca Valley, Calif., in 1992 jolted local residents at the time and continues to shake out new scientific findings. Called the Landers earthquake, after the closest town, the event triggered a number of other afterquakes across fault lines into Yellowstone in Wyoming and southern Montana.Los Alamos National Laboratory researcher Paul Johnson said this morning that he began reading about Landers quake about five years ago, but he was not satisfied with the explanations that were offered. When a colleague in Paris agreed with his hypothesis that a higher amplitude seismic wave might have been propagated that triggered quakes at a distance, he began serious work on the idea, but realized he needed experimental evidence.In a letter that appeared Thursday in the science journal Nature, Johnson and his colleagues, approaching the subject from a variety of disciplines, show how energy can be stored in certain types of granular materials like those found along fault lines worldwide. Each earthquake releases seismic waves or vibrations that travel through the Earth. The waves can trigger aftershocks miles away, and most aftershocks occur within hours to days after the main shock.Aftershocks from earthquakes can happen minutes, hours or even days after the sound waves that radiate from them pass, but the cause of the delay increases the mystery, a group of researchers have found.While researchers often assume that seismic waves beyond the immediate aftershock zone were too weak to trigger aftershocks, Johnson and his colleagues have proved that seismic activity sometimes increases thousands of miles away after an earthquake, according to an announcement by Los Alamos National Laboratory Thursday.Johnson said other earthquakes have since been identified that fit a similar profile, including the Dinali earthquake of 2002. That one was the largest inland quake in the United States in 150 years, running from Anchorage across Western Canada to Montana and Wyoming, and experiencing afterquakes for two years.“At these farther distances, earthquake triggering doesn’t happen all the time,” Johnson said. “The question always was why? What was going on in certain regions that led to triggering? The challenge was whether we could go into the laboratory and mimic the conditions that go on inside the Earth and find out.”The answer to the challenge was found at Pennsylvania State University, where Chris Marone developed a device that mimics earthquakes by pressing plates atop a layer of small glass beads. When enough energy is applied to the plates, they slip like tectonic plates about the mantle.Johnson wondered whether sound waves could induce earthquakes in such a system.The experiment found that when sound waves were applied for a short period of time just before the quake, they could induce smaller quakes. In some instances, the sound waves delayed the next earthquake.The sound waves appeared to affect earthquake behavior for as many as 10 earthquake events after they were applied. The team also found that the granular beads could store memory even after the system had undergone an earthquake and the beads rearranged themselves.“The memory part is the most puzzling, because during an earthquake, there is so much energy being released, and the event is so violent that you have to wonder, ‘Why doesn’t the system reset itself?” Johnson said.Many systems can demonstrate catastrophic failure based on a forced effect, like an earthquake. Other examples are avalanches and the collapse of sand dunes or a spacecraft that suffers continual vibration but cracks under greater turbulence.“What I’m wondering – and these are only thoughts – is can we come up with an overarching model to describe and predict catastrophic or breakdown behaviors,” Johnson said.The recent experiments conducted with the help of researchers at the University of California-Santa Cruz, the University of Wisconsin, the U.S. Geological Survey and the University of Washington, he said, has helped confirm that earthquakes are periodic events and that sound can disrupt them.

The Associated Press contributed to this report.