What causes earthquakes and how do they form?

Earthquakes are caused by the movement of tectonic plates, the large slabs of rock that make up Earth's outer shell. These plates are constantly moving, and when they collide, pull apart, or slide past each other, stress builds up along fault lines. When that stress exceeds the strength of the rock, the plates suddenly slip, releasing enormous amounts of energy in the form of seismic waves that travel through the ground. The point underground where the rupture begins is called the focus or hypocenter, and the point directly above it on the surface is the epicenter. Most earthquakes occur at tectonic plate boundaries, though they can also happen along interior fault lines. Volcanic activity and human activities like mining or reservoir construction can also trigger smaller earthquakes.