What Reid concluded from his investigations is illustrated in Figure 8. 5 Tectonic stresses acting over tens to hundreds of years slowly deform the crystal rocks on both sides of a fault. When deformed by differential stress, rocks bend and store elastic energy, much like a wooden stick does if bent (figure 8. 5B). Eventually, the frictional resistance holding the rocks in place is overcome. Slippage allows the deformed (strained) rock to "snap back" to its original, stress-free shape (figure 8. 5C, D). The "springing back" was termed elastic rebound by Reid because the rock behaves elastically, much like a stretched rubber band does when it is released. The vibrations we know as an earthquake are generated by the rock elastically returning to its original shape.