Elastic deformation and elastic rebound are two fundamental concepts explaining how earthquakes occur along faults in the Earth’s crust. These processes form the core of the elastic rebound theory, which was developed after the 1906 San Francisco earthquake. While both relate to the mechanical properties of rock, they represent distinct stages in the ongoing cycle of strain buildup and release driven by plate tectonics.
Understanding Elastic Deformation
Elastic deformation is the temporary change in the shape or volume of a rock body in response to applied stress. Tectonic plate movement generates immense forces—compression, tension, and shear—that act on rock masses near fault zones. Instead of immediately breaking or sliding, the crustal rock resists this force and begins to slowly bend or warp, much like a stretched rubber band.
This bending is classified as elastic because it is reversible; if the stress were suddenly removed, the rock would return to its original, undeformed shape. During this gradual process, the rock stores potential energy, often referred to as elastic strain energy, within its structure. The rate of deformation is slow, accumulating over months, years, or even centuries, corresponding to the relentless movement of tectonic plates.
Rock material can only withstand a certain amount of stress before its internal structure gives way. This maximum limit is known as the elastic limit or yield strength. As long as the rock remains below this limit, the deformation is purely elastic, and the energy is safely stored. Once the stress exceeds this threshold, the rock cannot maintain its integrity and must either undergo permanent plastic deformation or fracture.
Understanding Elastic Rebound
Elastic rebound is the mechanism by which the rock suddenly releases the energy stored during the deformation phase. The process begins when the accumulated stress finally surpasses the frictional strength locking the fault surfaces, or when the rock reaches its elastic limit and fractures. This failure is abrupt, triggering a rapid slip along the fault line.
The stored elastic strain energy is released in an instant, causing the rock on both sides of the fault to “snap back” to its original, undeformed shape. This sudden movement is the earthquake itself. The released energy travels outward from the point of rupture, known as the focus, in the form of seismic waves that cause ground shaking.
The concept of elastic rebound was first proposed by Harry Fielding Reid after observing the displacements caused by the 1906 San Francisco earthquake. He noted that physical features, such as fence lines, were offset across the fault, illustrating how the strained ground had recoiled to its previous position. Thus, the rebound is the dynamic event of energy discharge and the resulting motion.
The Critical Difference and Sequential Relationship
The fundamental difference between the two concepts lies in their role within the earthquake cycle: elastic deformation is the process of energy storage, and elastic rebound is the event of energy release. Deformation is a gradual, slow buildup of strain over an extended period. In contrast, rebound is an instantaneous, sudden action that occurs in a matter of seconds once the rock fails.
Deformation describes the state of the rock prior to the earthquake, where the rock is bent and holding potential energy. Rebound describes the physical movement and energy conversion during the earthquake. The rock’s change in shape during the inter-seismic period is the deformation, and the rock’s return to its original shape during the co-seismic period is the rebound.
Tectonic forces continuously drive the plates, causing elastic deformation to steadily accumulate strain energy in the rocks adjacent to a locked fault. This continues until the rock strength is exceeded, which triggers the elastic rebound. The rebound releases the accumulated stress through fault slip and seismic waves, effectively resetting the system and starting the cycle anew.