Earthquakes happen when underground rocks suddenly break and slip along a fault due to the buildup of stress and elastic strain energy caused by tectonic forces. This sudden release of energy creates seismic waves that make the ground shake. The location underground where the rock breaks is called the focus, and the point on the surface above it is the epicenter. Most earthquakes occur because tectonic plates slowly move but get stuck at their edges due to friction until the stress is great enough to overcome it, causing a rapid slip along the fault and the release of energy.

The process can be understood by imagining two blocks of rock stuck together that deform slowly until they suddenly slip past each other, releasing stored energy as an earthquake. The types of faults involved include strike-slip (where blocks slide horizontally), normal, and reverse faults (vertical movement). Earthquakes also produce different seismic waves (P, S waves) that radiate from the focus.

In summary, earthquakes happen due to the sudden rupture of rocks under stress along faults caused by tectonic plate movements, releasing stored elastic energy in the Earth's crust as shaking felt on the surface.