An earthquake is a sudden, rapid shaking of the ground caused by the release of energy in the Earth's lithosphere that creates seismic waves. Earthquakes can range in size from those that are so weak that they cannot be felt to those violent enough to toss people around and destroy whole cities. The severity of the shaking is measured using the Modified Mercalli Intensity Scale.
Earthquakes are usually caused by movement of the Earth's crust along a fault plane. A fault is a break in the Earth's crust, along which rocks can move. The Earth's crust is made up of several large tectonic plates that move around on the surface of the planet. Earthquakes often happen when two plates collide. The force of the collision creates a sudden release of energy that causes the shaking.
Earthquakes can also be caused by volcanic activity, landslides, and even man-made activities like explosions.