Earthquakes are caused by a sudden release of energy within the crust of the earth that extends to the surface. The internal earth is made up of layers. The top layer, the crust, is composed of large plates of rock that lie on top of the mantle, which is comprised of extremely hot, dense rock. Temperature differentials within the mantle produce convection currents that cause the mantel to flow. This ultimately results in movement in the crustal plates. Fractures, called faults, abound within the plates of the crust. As the plates move, the areas around the plate boundaries and the fault lines also move, causing rocks to rub together and catch. As movement continues, pressure builds up along these areas where the rocks are pressed together. Eventually, as movement continues and pressure builds, the rocks slip and jerk apart or break, releasing a sudden burst of energy that moves through the crust in waves. When these waves reach the surface of the earth, an earthquake results.
While most earthquakes are attributed to plate movement, human activities that involve the manipulation of the underground environment may contribute to some seismic occurrences. However, regardless of the source of the initial causal factor, an earthquake is the result of a sudden release of energy waves that extend to the surface due to sudden movement along plate boundaries or faults.
http://www.geo.mtu.edu/UPSeis/why.html http://volcano.oregonstate.edu/book/export/html/1082 https://earthquake.usgs.gov/ https://news.stanford.edu/2015/12/16/natural-manmade-quakes-121615
Earthquakes happen as a result of the movement of Earth's tectonic plates.
Earth's outer most layer is the lithosphere. It is composed of all of the crust and the upper, more rigid layer of the mantle. The lithosphere is divided into tectonic plates that move in relation to each other. Some plates move apart from each other, some move toward each other, and some move past each other. The place where two tectonic plates meet is called a boundary. A boundary where two plates move apart is called a divergent boundary. A convergent boundary occurs when two plates move toward each other, and a transform boundary occurs when two plates slide past each other.
The movement of the plates is caused by the interaction of several factors. As one plate slides beneath another plate, it sinks. This is called subduction. As the plate subducts at one end, it pulls the rest of the plate with it. This is called slab pull. At other boundary locations, magma might be pushing up through the boundary. This creates a bit of a ridge. As the magma at the ridge cools, it becomes denser and wants to sink. It essentially slides down the ridge and pushes the rest of the plate with it. This is called ridge push. Convection currents are also at play beneath the tectonic plates. These swirling currents of heated material also cause plate movement.
As the plates interact with each other, their movements cause stress along the plate edges. The stress forces can either be tension (pulling) or compression (pushing) forces, and rock layers will bend due to the stress being applied; however, rock layers can only bend so far or handle so much stress. Eventually, the stress buildup is so great that the energy is rapidly released. The plates rebound off of each other or apart from each other. It is that sudden rebounding movement that causes earthquakes. It's similar to a rubber band that you slowly keep stretching out. The energy in the elastic is built up slowly, but when it reaches the breaking point, it releases all of that energy in a very quick and violent way. While you could say that earthquakes are caused by the release of stored energy within the crust, that energy would not have been stored in the first place if Earth's lithosphere was completely static.
http://www.classzone.com/vpg_ebooks/ml_earthscience_na/accessibility/ml_earthscience_na/page_181.pdf
https://earthquake.usgs.gov/earthquakes/events/1906calif/18april/reid.php
https://www.worldatlas.com/aatlas/infopage/tectonic.gif
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