Final answer:
The stress required for a rock to slip along an existing fault is called yield stress or residual stress. Breaking stress refers to the point at which a rock fractures and forms a new fault, whereas yield stress is the lower stress needed for movement along an existing fault.
Step-by-step explanation:
The stress required to initiate movement on an already existing fault is called the yield stress or residual stress. Applying stress to a rock leads to deformation, which is the strain experienced by the rock. Initially, rocks behave in an elastic manner, returning to their original shape after the stress is removed. However, once the rock reaches its elastic limit, it deforms plastically or fracture, marking the loss of its ability to regain its previous shape upon unloading. The stress amount that a rock can tolerate before it yields or fractures differs based on whether a fault is new or pre-existing. Rocks have a higher breaking stress or ultimate stress for creating a new fault compared to the lower yield stress needed for slip along a pre-existing fault.
Understanding the difference between breaking stress and yield stress is crucial in geology, as it helps in predicting and understanding the geological events such as earthquakes and mountain building, which are associated with fault movements and rock deformation. The breaking stress is the point beyond which a rock will fracture, leading to the formation of a new fault. On the other hand, yield stress is the lower level of stress required for a rock to slip along an existing fault.