Definition - What does Earthquakes mean?
Earthquakes are sudden releases of energy that occur within the earth's crust or upper mantle and are caused, primarily, by movement along a fault line where the earth's tectonic plates meet one another, or volcanic activity. This causes seismic waves as energy is transmitted from the area of activity and through the earth. These waves are what cause the earth to shake.
Safeopedia explains Earthquakes
Earthquakes usually happen when rocks under the earth along a fault line break under pressure and the energy must go somewhere. Some of these occurrences are so powerful that they can cause widespread destruction. This usually happens when two plates are moving against one another and may stick due to friction. There are many areas where earthquakes happen frequently as they are along fault lines. An example is San Andreas.