Earthquakes are primarily caused by the movement of tectonic plates. These plates rest on the semi-fluid asthenosphere beneath them and can shift due to various geological processes. When the stress on the rocks exceeds their strength, they fracture along faults, releasing energy in the form of seismic waves. This process is known as elastic rebound, and it often leads to the ground shaking associated with earthquakes. Other contributing factors can include volcanic activity, human activities like mining, and the collapse of subterranean voids.