An earthquake (or quakes, tremors) is shaking caused by sudden movements of rocks in the Earth's crust. They can be extremely violent.
Earthquakes are usually quite brief, but may repeat. They are the result of a sudden release of energy in the Earth's crust. This creates seismic waves, which are waves of energy that travel through the Earth. The study of earthquakes is called seismology. Seismology studies the frequency, type and size of earthquakes over a period of time.
There are large earthquakes and small earthquakes. Large earthquakes can take down buildings and cause death and injury. Earthquakes are measured using observations from seismometers. The magnitude of an earthquake, and the intensity of shaking, is usually reported on the Richter scale. On the scale, 3 or less is scarcely noticeable, and magnitude 7 (or more) causes damage over a wide area.
An earthquake under the ocean can cause a tsunami. This can cause just as much death and destruction as the earthquake itself. Landslides can happen, too. This is an important part of the Earth's rock cycle.