Scientists at the University of Pennsylvania are studying a phenomenon called ageing that could help in predicting earthquakes — including when they will strike and how powerful they will be.

During ageing, static friction means that the longer that materials are in contact with each other, the more force is required to move them. Therefore, the longer something like a fault sits still, the more static friction builds up in the fault, causing it to become stronger.  

Faults that grow stronger over time create a large amount of stress and energy that 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,” said Robert Carpick, the John Henry Towne Professor and chair of the department of mechanical engineering and applied mechanics in Penn’s School of Engineering and Applied Science. “If you didn’t have ageing, then the fault would move very easily and 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.”

The Penn-based project takes a new look at the friction of rocks by studying rock friction at the nanoscale rather than the more commonly studied macroscale. Using this method, the researchers verified the first fundamental theory to describe ageing and describe what happens when the load increases.

The researchers conducted the study with silicon oxide, the primary component of many rock materials. They used a nanoscale tip made from silicon oxide to create force against a sample of silicon oxide, all under an atomic force microscope. They then varied the amount of normal force on the sample to determine how load affects the ageing behavior.

“That’s a very important question because load may have two effects,” Kaiwen Tian, Gr’17, a physics graduate in the School of Arts & Sciences, said. “If you increase load, you will increase contact area. It may also affect the local pressure.”

Previous research had suggested bonds could form more easily when more pressure was applied, but the new study found increasing the normal force actually just increases the amount of contact and the number of sites where atoms can react.

The researchers’ next step is to conduct the same study at very short timescales in order to learn more about the details of the energetics of the chemical bonds. This will show whether some bonds can form easily and if others take longer to form.

This work could lead to the creation of more efficient nano-devices due to an improved understanding of silicon friction. The researchers hope it also will lead to a better understanding of ageing, enabling better predictions of when earthquakes will occur. 

“Earthquake locations can be predicted fairly well,” Dr. Carpick said, “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 and, in the long term, we think these kinds of insights could help us predict earthquakes and other frictional phenomena better.”