What is an earthquake?

An earthquake is the sudden shaking of the ground caused by the movement of parts of the earth’s crust.

What causes earthquakes?

The outer layer of the earth is called the crust. The crust is broken up into several large pieces called crustal plates or tectonic plates. The diagram below shows the earth’s crustal plates. Notice how large they are.

_{Image by M. Bitton and used under this creative commons license.}

These plates move very slowly. They move only about one or two inches in a year. However, their movement sometimes causes great pressure to build up in the earth’s crust.  

When the pressure becomes too great it causes a sudden shift in the rocks along a fault, which is a crack in the earth’s crust. This sudden shift causes a release of energy as shock waves. When these shock waves reach the surface we feel them as an earthquake. Look at the video below which shows the effects of an earthquake in a home in Alaska.