An earthquake is a sudden shaking of the Earths surface caused by the movement of rocks deep underneath. Earthquakes occur when stresses in the Earths crust exceed the strength of the rock, causing it to break along lines of weakness, either a pre-existing or new fault plane. The point where an earthquake starts is called the focus or hypocenter and may be many kilometers deep within the Earth. The point at the surface directly above the focus is called the earthquake epicenter.

The energy released during an earthquake travels through the Earth in vibrations called seismic waves. These waves are what make the ground shake and can travel large distances in all directions. Near the focus, the waves can be very large, making them extremely destructive. The severity of an earthquake can be expressed in terms of both intensity and magnitude. Intensity is based on the observed effects of ground shaking on people, buildings, and natural features, while magnitude is a measure of the energy released by an earthquake.

Earthquakes can cause property damage, injury, and loss of life. They can also cause fires, tsunamis, landslides, and volcanic eruptions. The risks that earthquakes pose to society, including death, injury, and economic loss, can be greatly reduced by better planning, construction, and mitigation practices before earthquakes happen, and by providing critical and timely information to improve emergency response.