2 GLG345 Structural Geology Lecture 5 5.1.
Forces have magnitude and direction and are therefore vector quantities.
So, opposing forces must exist on opposite sides of the surface such that all forces on the surface sum to zero. The surface stress consists of a pair of equal and opposite tractions, or pair of equal and opposite traction components acting on a surface. The shear stress may be clockwise or anticlockwise. Structural Geology .