What theory explains earthquakes as the sudden release of strain built up by locked faults?

Elastic rebound theory explains earthquakes as the sudden release of strain built up by locked faults. It was developed after the 1906 San Francisco earthquake, when geologist Harry Fielding Reid studied ground displacement along the San Andreas Fault.

The theory compares a fault system with a bent elastic object. Tectonic forces gradually deform rocks on both sides of a locked fault. When the accumulated stress exceeds the rocks’ strength or frictional resistance, the fault slips, and the rocks spring toward a less-deformed shape. The released energy travels outward as seismic waves.

Elastic rebound does not mean every earthquake involves exactly one simple rupture. Faults can branch, rupture in segments, or creep slowly between earthquakes. Earthquake recurrence also varies because stress, friction, fluid pressure, and fault geometry differ from place to place.

A common mix-up is confusing elastic rebound with plate motion itself. Plate motion supplies the long-term stress, while elastic rebound describes the sudden earthquake-producing response.