The San Andreas fault was formed by the movement of the North American and Pacific tectonic plates sliding past each other in opposite directions. This movement causes displacement of objects on each side of the fault as stress from the movement builds up.
Because of its horizontal direction of movement, the San Andreas fault is categorized as a transform or strike-slip fault. Other faults are categorized by vertical movement. If the vertical movement is downward, the fault is categorized as a normal fault. Vertical movement upward creates a reverse fault.
The horizontal movement of the North American and Pacific tectonic plates is evidenced by many landforms along the San Andreas fault. Displacement of these landforms delineates the side-by-side passage of the plates. The San Andreas fault is rimmed by many other fissures and cracks in the Earth's surface, creating dips and valleys that follow the movement of the main fault line.
Some areas of the fault creep continuously on a daily basis, moving a few inches each year. Other segments are locked in place, building up pressure until the pressure erupts in an earthquake. Major earthquakes resulting in loss of life, and millions of dollars in damages have occurred along the fault line.