Earthquakes are caused by the sudden release of energy that has been stored in the Earth's crust. This energy is built up over time as the tectonic plates move and interact with each other. This movement is slow and steady, but when the pressure between the plates reaches a certain point, the plates suddenly slip, releasing the stored energy in the form of seismic waves. These waves travel through the Earth's crust, causing the ground to shake and resulting in an earthquake.