Byerlee's law

Byerlee's law is a scientific rule that explains how much shear stress (sideways force) is needed to make one rock surface slide over another. This depends on how strongly the rocks are being pressed together, which is called normal stress. The law is very important in geophysics and structural geology. It helps scientists understand how faults (cracks in the Earth’s crust) move and how earthquakes start.

The law is named after James D. Byerlee, an American scientist who studied this in the 1960s and 1970s. He did many lab experiments with different kinds of rocks and under different pressure conditions. From this, he came up with a rule that can be used to predict when rocks will start to slide.

Byerlee’s law is written in two parts, depending on how big the normal stress is:
 If the normal stress is less than 200 megapascals (MPa): Shear stress (τ) = 0.85 × normal stress (σₙ)
 If the normal stress is more than 200 MPa: Shear stress (τ) = 50 + 0.6 × normal stress (σₙ)

In these formulas:
 τ (tau) is the shear stress needed for sliding
 σₙ (sigma-n) is the normal stress pushing the rocks together

These equations show that rocks have a fairly steady friction level, usually between 0.6 and 0.85. The second formula adds 50 MPa to account for extra strength (called cohesion) that happens under higher pressure.

Byerlee's law is important because it is simple and works in many situations. Even though it was based on small lab tests, it fits what we see in real faults quite well. Scientists use this law in computer models to study how stress builds up in the Earth, how earthquakes happen, and how safe or dangerous certain areas might be. The law is also a key part of a bigger idea called the Mohr–Coulomb failure criterion, which helps predict when rocks will break or slide.

Byerlee's law is especially useful for understanding tectonic earthquakes, which happen when stress on a fault gets strong enough to overcome friction, causing a sudden slip and shaking the ground. The law is also used in mining, oil and gas drilling, and in studying human-caused earthquakes (like those from injecting water underground or using fracking). Over the years, scientists have added new ideas to improve Byerlee's law. These include the effects of temperature, water pressure, fault materials, and slow movements like creep. But the original version of the law is still widely used because it works well for many types of rocks, including granite, basalt, sandstone, and shale.

References

Category:Laws of physics