An earthquake is caused by the sudden movement of tectonic plates deep below the earth's surface. Tectonic plates are constantly slowly moving and putting pressure on each other. When the pressure exceeds the strength of the rocks, it causes the plates to suddenly slip past each other or collide, releasing energy in the form of seismic waves. Fault lines mark the boundaries between tectonic plates and are where most earthquakes occur. The theory of plate tectonics explains how the movement and interaction of these massive plates results in earthquakes and volcanic activity around the world.