We know tectonic plates move, but the real story of earthquakes lies in the forces driving that movement. The Earth's crust isn't just passively drifting; it's constantly being squeezed, stretched, and twisted. Geologists quantify these forces using the concept of stress, which is the force applied per unit area. At any point within the crust, stress acts in all directions, but we can simplify this complex picture by identifying three principal stress axes that are mutually perpendicular.

The orientation of these axes and their relative magnitudes determine how the rock will eventually fail and which type of fault will form. This fundamental relationship is the basis of Andersonian fault theory and allows us to connect the invisible forces within the Earth to the visible fault lines on its surface.