The elastic rebound theory is the leading geological model used to explain the cause of most earthquakes, describing a cycle of energy storage and sudden release within the Earth’s crust. This mechanism fundamentally explains why rock masses on either side of a fault move suddenly after a long period of being locked in place. The theory was first introduced by American geologist Harry Fielding Reid in 1910, following his detailed investigation of the catastrophic 1906 San Francisco earthquake and the resulting ground displacement along the San Andreas Fault. Reid’s work shifted the scientific understanding of earthquakes from simple rock fractures to a dynamic, cyclical process of accumulating and relieving strain.
The Slow Accumulation of Strain
The process begins with the continuous, slow motion of tectonic plates, which generates immense pressure on the surrounding rock masses. This force, known as tectonic stress, is the push, pull, or shear force applied across the rock body.
The rocks in the Earth’s brittle upper crust, especially near a fault line, are not infinitely strong and begin to change shape in response to this stress. The resulting change in the shape or volume of the rock is called strain, which represents the deformation of the material.
Because the rocks behave elastically, similar to a stretched rubber band, they resist this deformation and store the energy internally as elastic potential energy. Along active faults, the surfaces are often locked together due to friction, preventing a smooth, continuous slip. This locking forces the surrounding crust to bend and deform over vast distances and long periods, sometimes centuries. The strain accumulation rate is slow, typically only a few centimeters per year, which matches the rate of far-field plate movement.
The Sudden Release of Stored Energy
The strain accumulation phase continues until the rock mass reaches its elastic limit, also known as the rupture point. At this point, the stored energy is so great that the stress overcomes the frictional resistance and the internal strength of the rock. The sudden failure causes the rock to fracture, or for an existing fault to slip, initiating the earthquake.
The “rebound” is the instantaneous snapping back of the deformed rock on both sides of the fault to their original, undeformed shape. This sudden, rapid movement occurs over a matter of seconds, releasing the accumulated elastic potential energy. The energy is converted into kinetic energy, which propagates outward from the point of rupture, or hypocenter, in the form of seismic waves.
These seismic waves are what cause the ground to shake violently, and the magnitude of the earthquake is directly related to the amount of stored strain energy released.
Elastic Rebound and Fault Movement
The elastic rebound mechanism is confined to active fault zones, which are the boundaries between crustal blocks where relative movement occurs. This process applies to all major fault types, regardless of the direction of slip.
For a strike-slip fault, like the San Andreas Fault, the blocks move primarily sideways, while a thrust fault involves one block moving up and over another, and a normal fault involves downward movement. In each case, the rock adjacent to the locked fault bends and stores strain energy in the interseismic period, which is the time between earthquakes.
When the fault ruptures, the sides spring back, resulting in a sudden offset that can be observed on the surface or detected with modern instrumentation like GPS. Assessing seismic hazard potential in a given region relies on understanding the elastic rebound cycle. Seismologists use geodetic data to measure the current rates of strain accumulation across known faults. By calculating these strain rates, scientists can estimate how quickly a fault is re-loading and the potential size and recurrence interval of future earthquakes.