When the forces pushing on fault blocks cannot move one block past the other, potential energy is stored up in the fault zone. This is the same potential energy that resides in a giant boulder when it is poised, motionless, at the top of a steep slope. If something happens to overcome the friction holding the boulder in place, its potential energy will convert into kinetic energy as it thunders down the slope. In the fault zone, the potential energy builds up until the friction that sticks the fault blocks together is overcome. Then, in seconds, all the potential energy built up over the years turns to kinetic energy as the rocks surge past each other.