An earthquake is the result of a sudden release of energy in the Earth's crust that creates seismic waves.
At the Earth's surface, earthquakes manifest themselves by a shaking and sometimes displacement of the ground. When a large earthquake epicenter is located offshore, the seabed sometimes suffers sufficient displacement to cause a tsunami. The shaking in earthquakes can also trigger landslides and occasionally volcanic activity.
Earthquakes are caused mostly by rupture of geological faults, huge amounts of gas migration, mainly methane deep within the earth, but also by volcanic activity, landslides, mine blasts, and nuclear experiments.
It happens when two enormous pieces of rock (the earth's crust, usually continents) with fault lines try to rub past each other but end up catching on each other while they are sliding past each other (example of two pieces of rock sliding past each other:
-- _ ). When they catch on each other, they continue to push each other, guessing that they can slide past each other. The force of them pushing each other ( > < ) causes the rocks to break and the breaking ( >^< example of how the rocks look after they push too hard into each other) from the force of pressure on the rocks (the rocks pushing into each other) causes an earthquake.