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Earthquakes occur when stress that has been building up along a fault — a fracture in Earth's crust where two blocks of rock can slide past each other — is suddenly released. The driving force is plate tectonics: Earth's lithosphere is divided into large plates that move a few centimetres per year, driven by convection currents in the underlying mantle. Where plates meet, they either collide (convergent boundaries), move apart (divergent boundaries), or slide past each other horizontally (transform boundaries). At transform faults like California's San Andreas Fault, friction locks the plates together even as the underlying motion continues, building up elastic strain energy over decades or centuries. When friction is overcome and the blocks suddenly slip, elastic energy is released as seismic waves that travel outward from the focus (the point of rupture) in all directions, shaking the ground at the surface. The epicentre is the point on the surface directly above the focus.
answered by Omniscientia Team · 162 words · 18 Mar 2026