
A sudden slip on a fault caused by accumulated stress, releasing seismic energy.An earthquake is a sudden, rapid shaking of the ground caused by the release of accumulated stress along a geological fault. Over time, tectonic forces slowly deform the rocks on either side of a fault, storing elastic energy like a stretched spring. When the stress exceeds the frictional strength of the fault, the rocks slip abruptly, converting the stored energy into seismic waves that radiate outward and shake the surface.
The modern understanding of earthquakes as elastic rebound was developed by Harry Fielding Reid in 1910, following the 1906 San Francisco earthquake. He proposed that accumulated elastic strain is suddenly released when the fault slips.
The process begins with slow tectonic loading that builds stress across a fault. The fault's surfaces are held together by friction, which resists sliding. As stress increases, the fault may initially creep or stick in a series of small events. Eventually, the shear stress overcomes the static friction, causing a sudden rupture. The rupture propagates along the fault at kilometers per second, releasing energy as seismic waves. The behavior of the fault—whether it slips smoothly or in a stick-slip manner—depends on the frictional properties of the materials and the presence of fluids or gouge (crushed rock) in the fault zone.
Seismic hazard assessment and building design codes use earthquake mechanics to estimate ground shaking and design structures that can withstand it. Understanding fault friction also helps in interpreting induced seismicity from activities like wastewater injection.
The friction in a fault can be so sensitive that a tiny change in stress—equivalent to the weight of a coin—can trigger a large earthquake if the fault is already near its critical state.










