The elastic rebound theory posits that an earthquake is the result of a sudden release of elastic energy stored in the crust of the earth. Force and shifts in rocks on either side of a fault cause distortion and displacement until the pressure suddenly releases in the earthquake. An analogy is the elastic energy is a stretched rubber band that releases when someone breaks or cuts it.
Harry Fielding Reid first proposed the elastic rebound theory through observing the patterns of ground displacement over the San Andreas Fault after the devastating San Francisco earthquake in 1906. Reid used data from the U.S. Geological Survey to examine the movement of land along the California coast in the 50 years prior to the earthquake, and he determined the stress that built up along the fault resulted in the earthquake.
Although specific circumstances are often more complex than Reid envisioned, his theory stands up to modern GPS measurements. Geologists recognize that most earthquakes occur due to elastic rebounding of stored energy.
Scientists illustrate the phenomenon by imagining a road or fence built over a fault line. As pressure builds up in the locked plate boundaries, the fence distorts into an S pattern, which may happen only a few inches a year over the space of many years. Once the stress becomes too great, the locked rocks spring back, the earthquake occurs and the fence breaks. The locked rocks may take hundreds of years to reach the point of maximum stress and only seconds to release the pent-up energy.