Earthquake, any sudden shaking of the ground caused by the passage of seismic waves through Earth’s rocks. Seismic waves are produced when some form of energy stored in Earth’s crust is suddenly released, usually when masses of rock straining against one another suddenly fracture and “slip.”
Earthquakes occur most often along geologic faults, narrow zones where rock masses move in relation to one another. The major fault lines of the world are located at the fringes of the huge tectonic plates that make up Earth’s crust.
Major earthquakes, some generating tsunamis, have leveled entire cities and affected whole countries. Relatively minor earthquakes can also be induced, or caused by human activity, including extraction of minerals from Earth and the collapse of large buildings.