San Andreas fault line in California. (Source: © oliver de la haye / stock.adobe.com)

Planar fracture of rock, where compressional or tensional forces cause relative displacement of the rocks on the opposite sides of the fracture. Geology recognizes several types of faults according to the way the rocks move relative to each other. The largest fault lines are at lithospheric plate boundaries. Plate movement along fault lines is the source of earthquakes.