A fault is a crack or fracture in the Earth's crust where rocks on either side have moved relative to each other. Faults can range in size from small, barely noticeable cracks to large, major fault lines that extend for miles. Faults form due to the tectonic forces and movement of the Earth's lithospheric plates. When stress builds up along a fault line and becomes too great for the rocks to withstand, it is released in the form of an earthquake.
