Using single calcite crystals with varying surface roughness allows engineers to simplify the complex physics that describes fault motion. In a new study from the University of Illinois at Urbana-Champaign, researchers show how this simplification can lead to better earthquake prediction.
Scientists describe the behavior of faults using models based on observational studies that take into account the friction coefficients of rocks and minerals. These “rate and state” equations calculate the strength of the fault, which has implications for the drive and frequency of earthquakes. However, applying these empirical models to earthquake prediction is impractical 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, takes a look at the relationship between friction and surface roughness of calcite – one of most common rock-forming minerals in the earth’s crust – to formulate a more theoretical approach to defining the laws of rate and state.
Results are published in the Proceedings of the National Academy of Sciences.
“Our goal is to examine nanoscale processes that can trigger fault movement,” said Binxin Fu, CEE graduate student and first author of the study. “The processes we study at the nanoscale are less complex than the processes at the macroscopic scale. For this reason, we aim to use microscopic observations to bridge the gap between the nanoscale and macroscale worlds to describe defect behavior using less complexity.”
The roughness of a mineral crystal depends mainly on its atomic construction. However, the researchers said the rocks in the get hold of areas are scraped, dissolved and annealed as they rub against each other, also affecting their texture at the nanoscale.
To test how nanoscale mineral roughness can affect fault behavior, the team prepared atomically smooth and rough calcite crystals in dry and wet environments to simulate dry rocks and those containing pore water. Atomic drive microscopy measured friction by sliding a tiny pressure-mounted silicon tip over different crystalline surfaces exposed to simulated fault zone situations: wet area and smooth calcite wet area and rough calcite surface dry area and smooth calcite and floor dries with rough calcite.
“Friction may increase or decrease with slip speed as a function of mineral types and environment,” Espinosa-Marzal said. “We found that in calcite, friction generally increases with sliding velocity along rougher mineral surfaces – and even more so in the presence of water. By using data from such a common mineral fate 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 their experimental results to studies performed in natural environments with rocks containing calcite at shallow crustal levels.
“Our results are consistent with a recent study showing that water decreases fault strength compared to dry conditions,” Espinosa said. -Marzal. “Our results are also consistent with another study showing that low-frequency earthquakes tend to occur along wet faults, suggesting that a decrease in friction – caused by water – may be a mechanism of slow earthquakes in certain environments.”
This breakthrough may help seismologists redefine the rate and state laws for determining where stress builds up in the crust – and give clues to where and when future earthquakes could occur.
The team recognizes that there are still many other factors to consider, including the temperature and the influence of other common crustal minerals such as quartz and mica. The researchers plan to incorporate these variables into future models.
Countrywide Science Basis supported this study.