The San Andreas Fault runs through the western part of California, and under the Pacific Ocean. This fault is the meeting line for the North American plate and the Pacific plate. Both of these plates move in different directions and at different rates. The movement of the plates past one another is not at a steady rate. The plates often stick because their edges are not smooth. Pressure between the plates constantly increases until it becomes so great that the rocks break apart. This sudden release of energy creates an earthquake.