An earthquake is a sudden movement of the Earth, caused by the abrupt release of strain that has accumulated over a long time. For hundreds of millions of years, the forces of plate tectonics have shaped the Earth as the huge plates that form the Earth's surface slowly move over, under, and past each other. Sometimes the movement is gradual. At other times, the plates are locked together, unable to release the accumulating energy. When the accumulated energy grows strong enough, the plates break free. If the earthquake occurs in a populated area, it may cause many deaths and injuries and extensive property damage.
Small earthquakes may be set off by landslides, volcanoes or even just heavy traffic. Big earthquakes are set off by the grinding together of the vast tectonic plates that make up the Earth's surface.
Tectonic plates are sliding past each other all the time, but sometimes they stick. The rock bends and stretches for a while and then snaps. This makes the plates jolt, sending out the shock waves that cause the earthquake's effects to be felt far away.
Tectonic plates typically slide 4 or 5 cm past each other in a year. In a slip that triggers a major quake they can slip more than 1 m in a few seconds.
In most quakes a few minor tremors (foreshocks) are followed by an intense burst lasting just one or two minutes. A second series of minor tremors (aftershocks) occurs over the next few hours.
The epicentre of an earthquake is the place on the earth's crust right above the centre of the earthquake. The centre is always at a certain depth in the earth and this centre is the actual place of the earthquake. The centre of an earthquake is also called hypocentre. Earthquakes are strongest at the epicentre and become gradually weaker farther away.
Certain regions called earthquake zones are especially prone to earthquakes. Earthquakes zones lie along the edges of tectonic plates.
A shallow earthquake originates 0-70 km below the ground. These are the ones that do the most damage. An intermediate quake begins 70 - 300 km down. Deep quakes begin over 300 km down. The deepest-ever recorded earthquake began over 720 km down.