How do earthquakes happen?

Earthquakes occur when stress on geological faults is suddenly released. Tectonic plates or rock masses can become locked due to friction while tectonic forces keep pushing. When accumulated strain exceeds frictional strength, the fault slips abruptly. This abrupt displacement sends out seismic waves through the Earth. Essentially, rocks elastically deform until they break; the sudden fracture and slip release energy as an earthquake. The point of slip (hypocenter) radiates shockwaves that we feel as shaking. Most quakes result from plate boundary motions (e.g. subduction zones, transform faults) releasing pent-up elastic energy.