Penn and Brown Researchers Demonstrate Earthquake Friction Effect At The Nanoscale

Earthquakes are some of the most daunting natural disasters that scientists try to analyze. Though Earth's major fault lines are well known, there is little scientists can do to predict when an earthquake will occur or how strong it will be. And, though earthquakes involve millions of tons of rock, a team of University of Pennsylvania and Brown University researchers has helped discover an aspect of friction on the nanoscale that may lead to a better understanding of the disasters. Robert Carpick, a professor who chairs the Department of Mechanical Engineering and Applied Mechanics in Penn's School of Engineering and Applied Science, led the research in collaboration with Terry Tullis and David Goldsby, professors of geological science at Brown. The experimental and modeling work was conducted by first author Qunyang Li, a postdoctoral researcher in Carpick's group, who has recently been appointed an associate professor in the School of Aerospace at Tsinghua University, China.

Their work will be published in the journal Nature.

The team's research was spurred by an unusual phenomenon that has been observed in both natural and laboratory-simulated faults: materials become more resistant to sliding the longer they are in contact with one another. This trait is actually fundamental to why earthquakes happen at all. The longer materials are in contact, the stronger the resistance between them and the more violent and unstable the subsequent sliding is. Energy is stored over the time the materials are in contact and is then catastrophically released as an earthquake.

While geologists, physicists and mechanics researchers have studied this phenomenon for decades, the mechanism behind this increase of friction over time has only been hypothesized. There are two main theories as to why this "frictional aging" occurs.

"One hypothesis is that points of contact deform and grow over time -- that contact quantity increases," Carpick said. "The other is that bonding at the points of contact strengthens over time -- that contact quality increases."

The difficulty in proving that either theory holds true lies in the fact that points of contact are necessarily embedded at the juncture of two materials and are therefore hard to observe. One of the original breakthrough experiments on these theories projected light through transparent materials held together to measure the growth of apparent contact points. While this lent credence to the contact quantity theory, there was not yet a way to assess the bond strengths at those individual points of contacts or to be sure that the observations were of single points of contacts or clusters of even smaller nanoscale contacts.