‘Ageing’ phenomenon causes powerful earthquakes, report Penn researchers

Scientists have gotten better at predicting where earthquakes will occur, but they are still in the dark about when earthquakes will strike and how devastating they will be.

In the search for clues, scientists at Penn are studying a phenomenon called “ageing.” In ageing, the longer materials are in contact with each other, the more force is required to move them. This resistance is called static friction. The longer an object, such as a fault, is sitting still, the more static friction builds up and the stronger the fault gets.

Even when the fault remains still, tectonic motion is still occurring. Stress builds up in the fault as the plates shift, until finally they move so much that they exceed the static friction force and begin to slide. Because the fault grew stronger with time, the stress can build up to large levels, and a colossal amount of energy is released in the form of a powerful earthquake.

“This ageing mechanism is critical in underlying the unstable behavior of faults that lead to earthquakes,” says Robert Carpick, the John Henry Towne Professor and chair of the Department of Mechanical Engineering and Applied Mechanics in the School of Engineering and Applied Science. “If you didn’t have ageing, then the fault would move very easily, so you’d get much smaller earthquakes happening more frequently, or maybe even just smooth motion. Ageing leads to the occurrence of infrequent, large earthquakes that can be devastating.”

Scientists have been studying the movement of faults and ageing in geological materials at the macroscale for decades, producing phenomenological theories and models to describe their experimental results. But there’s a problem when it comes to these models.

“The models are not fundamental, not physically based, which means we cannot derive those models from basic physics,” says Kaiwen Tian, a Ph.D. student in the Department of Physics & Astronomy in the School of Arts & Sciences and a member of Carpick’s lab.

A Penn-based project seeks to understand the friction of rocks from a more physical point of view at the nanoscale.

In their most recent paper, the researchers, led by Carpick and Tian, verified the first fundamental theory to describe ageing and explain what happens when load increases.

In addition to providing a better understanding of earthquakes, this work could lead to more efficient nano-devices. Due to the fact that many micro- and nano-devices are made from silicon, understanding friction is key to getting those devices to function more smoothly.

But, most importantly, the researchers hope that somewhere down the line, a better understanding of ageing will enable them to predict when earthquakes will occur.

“Earthquake locations can be predicted fairly well,” Carpick says, “but when an earthquake is going to happen is very difficult to predict, and this is largely because there’s a lack of physical understanding of the frictional mechanisms behind the earthquakes. We have a long way to go to connect this work to earthquakes. However, this work gives us more fundamental insights into the mechanism behind this ageing. In the long term, we think these kinds of insights could help us predict earthquakes and other frictional phenomena better.”