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.
