What are earthquakes?

Most people living in California have at one time or another experienced an earthquake. Without warning we wake up in the middle of the night because of the sometimes violent, sometimes slow rolling motion of the earth; other times we are driving on the freeway, walking in the mall, or even just watching television when an earthquake occurs. So what are earthquakes? What causes them? Why do earthquakes feel so different from one another?

Earthquakes are the Earth's natural means of releasing stress. When the Earth's plates move against each other, stress is put on the lithosphere. When this stress is great enough, the lithosphere breaks or shifts. Imagine holding a pencil horizontally. If you were to apply a force to both ends of the pencil by pushing down on them, you would see the pencil bend. After enough force was applied, the pencil would break in the middle, releasing the stress you have put on it. The Earth's crust acts in the same way. As the plates move they put forces on themselves and each other. When the force is large enough, the crust is forced to break. When the break occurs, the stress is released as energy which moves through the Earth in the form of waves, which we feel and call an earthquake.