Seismic activity is the movement of the Earth's crust caused by the release of energy from within the planet. It is usually associated with earthquakes, but can also include other phenomena such as volcanic eruptions and landslides. Two common forms of seismic activity are shocks and tremors. Shocks are sudden, violent movements of the Earth's surface that cause destruction and can be felt over large distances. Tremors are more subtle vibrations that can be felt over a smaller area.