Final answer:
When stress builds up on a fault, the rocks store elastic potential energy that can be released when the stress is removed.
Step-by-step explanation:
When stress builds up over many years on a fault, the rocks store elastic potential energy. This energy is stored when the rocks are deformed and can be released when the stress is removed, causing the rocks to return to their original shape. This behavior is similar to stretching a rubber band and then releasing it.