Earthquake - Students | Britannica Kids | Homework Help

Earthquakes are caused by the sudden release of energy within a limited region of the rocks of the Earth. Most earthquakes are caused by the movements of Earth’s tectonic plates, but earthquakes can also be caused by volcanic activity and even by human activities.

Major earthquakes occur mainly along the edges of Earth’s tectonic plates. The strongest and most destructive quakes are associated with ruptures, or breaks, in the Earth’s crust, which are known as faults. Pressures from within Earth strain the tectonic plates that make up the crust. The strain builds until suddenly the plates move along the faults, thereby releasing energy. The plates slip and slide in opposite directions along this fracture in the rock, shaking the ground above. The masses may move up and down or sideways. On Earth’s surface, displacement of the ground may vary from a few inches to many feet (several centimeters to many meters).

Earthquakes can also be caused by volcanic activity. Some volcanic earthquakes occur when a fault underneath a volcano moves. In these cases, a fault slip can cause both an earthquake and a volcanic eruption, when magma is able to flow into a crack that opens between the tectonic plates. In other cases, magma rises between rock layers, creating a gap. If it flows back out of that gap, it leaves an empty space, which can collapse and create an earthquake. The movement of magma itself can also cause earthquakes as the magma pushes on layers of rock. Such earthquakes can be warning signs of a volcanic eruption. Earthquakes of this type helped predict the eruptions of Mount Saint Helens in Washington in 1980 and Mount Pinatubo in the Philippines in 1991.

A small proportion of earthquakes are caused by human activity. Dynamite or nuclear explosions, for example, can sometimes cause mild quakes. The types of earthquakes caused by nuclear explosions are slightly different from tectonic earthquakes. Monitoring for these kinds of earthquakes helps countries detect hidden nuclear explosions and enforce the nuclear test-ban treaty. Digging mines can cause earthquakes when the removal of rock causes slipping of existing faults or when a mine collapses. Large reservoirs contained by dams can also cause earthquakes. The massive amounts of water held back by a dam exert pressure downward on the rocks below. Tiny cracks in the rocks fill with water. In areas where there are existing faults, this high-pressure water can lubricate the fault, causing it to slip and create an earthquake. Fracking, a process of injecting liquids deep into the ground to crack the rock layers and extract natural gas, also causes earthquakes when these injected fluids lubricate existing faults.