Nanoscale observations simplify how scientists describe earthquake movement
By using single calcite crystals of varying surface roughness, engineers can simplify the complex physics describing fracture motion. In a new study from the University of Illinois Urbana-Champaign, researchers show how this simplification could lead to better earthquake prediction.
Scientists describe fault behavior using models based on observational studies that explain the friction coefficients of rocks and minerals. These “rate-and-state” equations calculate fault strength, which has implications for earthquake strength and frequency. However, applying these empirical models to earthquake prediction is not practical due to the number of unique variables that must be taken into account for each fault, including the effect of water.
The study, led by Rosa Espinosa-Marzal, professor of civil and environmental engineering, looks at the relationship between friction and the surface roughness of calcite — one of the most abundant rock-forming minerals in the Earth’s crust — to formulate a more theoretical approach to defining speed and state laws.
The findings are published in the Proceedings of the National Academy of Sciences.
“Our goal is to investigate the nanoscale processes that can cause fault movements,” said Binxin Fu, a CEE graduate student and the study’s lead author. “The processes we investigate at the nanoscale are less complex than at the macroscale. Therefore, we want to use microscopic observations to bridge the gap between the nanoscale and the macroscale world to describe error behavior with less complexity.”
The roughness of a mineral crystal mainly depends on its atomic structure. However, the researchers said the rocks in contact zones are scraped, dissolved and annealed as they rub against each other, which also affects their nanoscale texture.
To test how mineral roughness at the nanoscale can influence fracture behavior, the team prepared atomically smooth and rough calcite crystals in dry and wet environments to simulate dry rocks and those with pore water. Atomic force microscopy measured friction by dragging a small pressure-mounted silicon tip across various crystal surfaces exposed to simulated fracture zone conditions: wet surface and smooth calcite; wet surface and rough calcite; dry surface and smooth calcite; and dry surface with rough calcite.
“Friction can increase or decrease with sliding speed, depending on the mineral species and the environment,” Espinosa-Marzal said. “We found that in calcite, friction typically increases with sliding speed along rougher mineral surfaces – and even more in the presence of water. By using data from such a common mineral type and a limited number of contact scenarios, we reduce the complexity of the analysis. and provide a fundamental understanding of rate-and-state equations.”
The team compared the experimental results with studies from natural environments of calcite-containing rocks at shallow crustal levels.
“Our results are consistent with a recent study showing that water lowers fault strength compared to dry conditions,” Espinosa-Marzal said. “Our findings are also consistent with another study showing that low-frequency earthquakes tend to occur along wet faults, suggesting that reduced friction — caused by water — may be a mechanism for slow earthquakes in some environments.”
These advances could help seismologists redefine speed and state laws to determine where stress builds up in the crust — and provide clues as to where and when future earthquakes may occur.
The team recognizes that there are many other factors to consider, including temperature and the influence of other common crustal minerals such as quartz and mica. The researchers plan to include these variables in future models.
Study provides new scale of understanding of earthquakes
Binxin Fu et al, Velocity weakening and amplifying friction in single and multi-asperity contacts with calcite monocrystals, Proceedings of the National Academy of Sciences (2022). DOI: 10.1073/pnas.2112505119
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