Earthquakes are due to the breaking of rock at depth. Although the Earth does not seem to move, in fact there are forces acting on rocks and these forces are called stresses. When the stress at some location exceeds the strength of the material rock, the rock fractures and THAT is an earthquake.

An earthquake happens when the stress of the rocks sliding past one-another becomes so great that friction can no longer keep them from moving. When this break happens, you get an earthquake as the rocks keep moving until friction can stop them again.

And earthquake is caused when pieces of the earth suddenly move, and they jerk the earth. Earthquakes are most common along tectonic plate boundaries, and at faults. For example, at the San Andreas fault, one side of the earth is moving in a different direction than the other. If a big amount of movement happens, one side jerks past the other and the ground shakes with the movement. Geologists and Geophysicists are the scientists who study earthquakes and they track how much movement occurs every year.