An earthquake is caused by the sudden release of energy due to movement along faults in the Earth's crust. This movement occurs when tectonic plates, which are slowly shifting, become stuck due to friction, causing stress to build up. When the stress is released abruptly, it causes the rock to break and slip, producing seismic waves that make the ground shake. This point underground where the rock first breaks is called the focus or hypocenter, while the point on the surface directly above it is known as the epicenter. Most earthquakes happen along fault lines where the Earth's tectonic plates interact, such as sliding past each other, colliding, or moving apart. Besides natural tectonic activity, earthquakes can also be triggered by volcanic activity or human activities like mining, reservoir filling, and fracking. The sudden slip along a fault releases stored elastic strain energy which propagates as seismic waves causing ground shaking.