Ø Over the course of time, one can observe that the two sides of an active fault are in slow but continuous movement relative to one another. This movement is known as fault slip.
Ø The rate of this movement may be as little as a few inches or so per year
Ø We can infer the existence of conditions or forces deep within the fault which resist this relative motion of the two sides of the fault.
Ø This is because the motion along the fault is accompanied by the gradual buildup of elastic strain energy within the rock along the fault.
Ø The rock stores this strain like a giant spring being slowly tightened.
Ø Eventually, the strain along the fault exceeds the limit of the rocks at that point to store any additional strain. The fault then ruptures--that is, it suddenly moves a comparatively large distance in a comparatively short amount of time.
Ø The rocky masses which form the two sides of the fault then "snap" back into a new position. This snapping back into position, upon the release of strain, is the "elastic rebound" of Reid's theory
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