If you've read How Earthquakes Work , then you know that the earth's surface is made up of large, rigid plates that slowly drift over the mantle layer below. At the boundaries between plates, a number of things can happen. This boundary forms a fault line that extends under the ocean and on land along the west coast of the United States. The San Andreas fault in California is the piece that's on land. Smaller faults form in the crust material near the boundary line due to the forces of the plates pushing on each other.
Friction builds up along faults because the two sides are pushed very tightly together.
When this happens, tension builds up along the fault line until the force of movement is great enough to overcome the force of friction. Many scientists estimate that there is enough tension built up along some locked California faults, that when they do finally slip, the earthquake will be extremely powerful. The Hayward Fault particularly concerns these scientists because it runs under heavily populated areas in and around Los Angeles. The notion that part of California will break off was likely inspired by the San Andreas fault. After all, since the fault goes right through California, one part of the state is on the Pacific plate and one is on the North American plate.