Earthquakes occur when two blocks of earth suddenly slip past each other. This sudden release of energy causes the seismic waves that make the ground shake. The surface where they slip is known as the fault or fault plane.
According to Universe Today, earthquakes occur as a result of tectonic plates colliding and volcanic eruption. To be considered an earthquake, a shock wave has to be of natural origin. Earthquakes occur when rock underground suddenly breaks along a fault. The spot underground where the rocks break is the focus of the earthquake. The place directly above the focus on top of the ground is the epicenter of the earthquake.
Major earthquakes are often preceded by more frequent minor shocks, known as foreshocks, as the rocks begin to move. Following the main shock, further movements, known as aftershocks, occur as the rock masses settle in their new positions. Aftershocks cause problems for rescue services because they can bring down buildings that were weakened by the primary earthquake.
Earthquake-like seismic waves can also be caused by explosions underground. These explosions are triggered in order to break rock while making tunnels for roads, railroads, subways or mines. These manmade explosions, however, don't cause very strong seismic waves.