Byerlee's law is an experimentally derived law in physics that gives the stress circumstances in the Earth's crust at which fracturing along a geological fault takes place. The relation was determined by American geophysicist James Byerlee, by using experimental data to solve the Mohr–Coulomb failure criterion.
In which is the shear stress and the normal stress. is the cohesion or internal strength of the material. The value is the pore fluid pressure inside a rock, which is constant on a small scale and weakens the rock. Byerlee found that in the upper crust, the criterion can be simplified to:
However, the crust is far from a homogeneous material and consists of many rock types. Material constants can therefore vary locally. Even though Byerlee's law is a simplification, it is a good enough approximation for almost all situations. Byerlee's law gets less accurate when pressures and temperatures get higher than normal in the upper crust (e.g. temperatures over 400°C)