Anderson's theory of faulting

[1][2] A fault is a fracture in the surface of the Earth that occurs when rocks break under extreme stress.

The grinding of two rock masses against each another along a fault results in an earthquake and deformation of the Earth's crust.

[4] Faults can be classified into four types based on the kind of motion between the separated rock masses: normal, reverse, strike-slip, and oblique.

Dip is defined as the angle of the fault relative to the surface of the earth, which indicates the plane on which slip will occur.

See animation here[5] Strike-slip faults occur when the blocks slide against each other laterally, parallel to the plane.

Shear stress acts in the direction against motion as two parallel planes move against one another.

[4] In Anderson's fault theory, the earth's surface is considered to be a collection of horizontal segments that connect to comprise one plane which experiences no shear stress parallel to itself.

The theory models faulting in terms of these three principal stresses denoted as σ₁, σ₂, and σ₃.

σ₂ is perpendicular to both σ₁ and σ₃ and in the cases of dip-slip and strike-slip faults will cause no motion in its direction, making it the second strongest principal stress.

[2] Anderson's theory classifies tectonic environments into three fault regimes based on their relationship with the principal stresses.