Earthquakes are caused primarily by the sudden release of energy in the Earth's crust, usually due to the movement and interaction of tectonic plates. This movement can occur along fault lines, which are fractures in the Earth's surface where stress builds up over time. When this stress exceeds a certain threshold, it is released as seismic waves, causing the ground to shake. Other factors can also induce earthquakes, such as volcanic activity and human activities like mining or reservoir-induced seismicity.