Nanoscale research could help us better understand how earthquakes happen

Microscopic friction measurements find some rocks can dissolve and may cause faults to slip

We could better understand how earthquakes work thanks to new nanoscale research which studies the relationships between water, friction and mineral chemistry.

This was completed by scientists at the University of Illinois, who used microscopic friction measurements to find that some rocks can dissolve and may cause faults to slip.

It's thought that the study, published in the journal Nature Communications, could lead to a better understanding of earthquake dynamics.

As part of the research, the scientists closely examined how water and calcite (a mineral that is very common in the Earth's crust) interact at various pressures and groundwater compositions to influence frictional forces along faults.

"Water is everywhere in these systems," said civil and environmental engineering professor, Rosa Espinosa-Marzal, who was a co-author on the study. "There is water on the surface of minerals and in the pore spaces between mineral grains in rocks. This is especially true with calcite-containing rocks because of water's affinity to the mineral."

The study focuses on calcite-rich rocks in the presence of naturally occurring salty groundwater, or brine, along fault surfaces. The rock surfaces that slide past each other along faults are not smooth. The researchers zoomed in on the naturally occurring tiny imperfections or unevenness on rocks' surfaces, called asperities, at which friction and wear originate when the two surfaces slide past each other.

"The chemical and physical properties of faulted rocks and mechanical conditions in these systems are variable and complex, making it difficult to take every detail into account when trying to answer these types of questions," added Espinosa-Marzal.

"So, to help understand water's role in fault dynamics, we looked at a scaled-down, simplified model by examining single asperities on individual calcite crystals."

For the experiments, the team submerged calcite crystals in brine solutions at various concentrations and subjected them to different pressures to simulate a natural fault setting. Once the crystals were in equilibrium with the solution, they used an atomic force microscope to drag a tiny arm with a silicon tip across the crystal to measure changes in friction.

The researchers found what they expected: As the pressure applied on the crystals increased, it became more difficult to drag the tip across the crystal's surface. However, when they increased pressure to a certain point and the tip was moved slowly enough, the tip began to slide more easily across the crystal.

This told them that something had happened to the tiny asperity under higher pressures that caused a decrease in friction. The atomic force microscope also allows them to image the crystal surface, and see that the groove increased in size, confirming that the calcite had dissolved under pressure.

While Espinosa-Marzal said that this shows that such studies warrant serious consideration in future work, she admitted that there are still many more questions related to the research that need to be answered..

"Our research also suggests that it might be possible to mitigate earthquake risk by purposely changing brine compositions in areas that contain calcite-rich rocks. This consideration could be beneficial in areas where fracking is taking place, but this concept requires much more careful investigation," she added.