The surface of the Earth moves all the time. This may not be obvious. Usually the plates of the Earth slide past, or under each other at a rate of one or two centimeters per year. But sometimes they get stuck. Then the forces driving the plates past each other build up. Eventually the forces become strong enough to force the plates apart. Then we have an earthquake. Earthquakes move sections of a plate several feet at once.
The picture to the left shows an example of a typical scar left in the crust from the motion of the earth. Such a scar, along which plates slide against each other is called a fault .