Nanoscale observations simplify how scientists describe earthquake motion

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The use of calcite single crystals with variable surface roughness allows engineers to simplify the complex physics describing fault motion. In a new study from the University of Illinois at Urbana-Champaign, researchers show how this simplification may 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 condition” equations calculate the 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 considered for each fault, including the effect of water.

The study, led by civil and environmental engineering professor Rosa Espinosa-Marzal, investigates the relationship between friction and the surface roughness of calcite — one of the most common rock-forming minerals in Earth’s crust — to formulate a more theoretical approach to determining the rate of state laws.

The results were published in the Proceedings of the National Academy of Sciences.

“Our goal is to examine nanoscale processes that may lead to faulty motion,” said Binxin Fu, a graduate student at CEE and first author of the study. “The processes we achieve at the nanoscale are less complex than the larger ones. For this reason, we aim to use microscopic observations to bridge the gap between the nanoscale and the scale worlds to describe erratic behavior using less complexity.”

The roughness of a mineral crystal mainly depends on its atomic structure. However, the researchers said that the rocks in the contact areas are scraped, melted and hardened as they rub against each other, which also affects their nanostructure.

To test how nanoscale mineral roughness can affect faulty behavior, the team prepared coarse and atomically smooth calcite crystals in dry and wet environments to simulate dry rocks and those with porous water. Atomic force microscopy measured friction by pulling a small pressure-tipped silicon tip across different crystal surfaces subjected to simulated fracture zone conditions: wet surface and smooth calcite; wet surface and coarse calcite; dry surface and smooth calcite; And dry with raw calcite.

“Friction can increase or decrease with slip speed depending on the types of metal and the environment,” Espinosa-Marzal said. “We find that in calcite, friction typically increases with the rate of slip along rough mineral surfaces—and even more so 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 basic understanding of rate and state equations. “.

The team compared their results to studies from natural environments with calcite-containing rocks at shallow crust levels.

“Our results are consistent with a recent study showing that water reduces fault resistance compared to dry conditions,” Espinosa-Marzal said. “Our results 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.”

This advance may help seismologists redefine the laws of rate and state to determine where stress builds up in the crust — and give clues to where and when earthquakes will occur in the future.

The team acknowledges that there are still many other factors to consider, including temperature and the influence of other common crust minerals such as quartz and mica. The researchers plan to incorporate these variables into future models.

Study produces a new metric for understanding earthquakes More information: Binxin Fu et al, Velocity weakening and friction strengthening on single and multiple contacts with calcite monocrystals, Proceedings of the National Academy of Sciences (2022). DOI: 10.1073/pnas.2112505119 Submitted by University of Illinois at Urbana-Champaign

Citation: Nanoscale observations simplify how scientists describe earthquake motion (2022, Aug. 2) Retrieved on Aug 2, 2022 from https://phys.org/news/2022-08-nanoscale-scientists-earthquake-movement.html

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