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Earthquakes come and go from the public eye with such global disasters as the Japanese Earthquake of 2011 and the Haitian earthquake of 2010. Despite large portions of the Earth's population living in potentially at-risk zones, earthquakes and their causes remain a mysterious concept to many. What does cause them? The short answer is that loose chunks of the earth's outer layer whack into each other, the long answer is as follows.

Making the ground shake like a puddle is no easy feat, and for noticeable earthquakes an enormous amount of energy is required. Where does this force come from? The origin of the majority of earthquakes is movement of the earth's tectonic plates.

The earth, rather than being a homogeneous ball of rock, consists of layers. As seen in this NASA illustration, there are four primary layers: the inner and outer cores at the earth’s center, and the inner and outer mantles above the core. Bordering the outer mantle is a thin layer of rock known as the earth's crust. You could think of these layers as those of a four level cake, with the crust being icing on top.

The tectonic plate movement that we are looking to understand results from interaction between the mantle and crust in a zone known as the lithosphere. The lithosphere is formed from the uppermost portion of the outer mantle and the crust taken together. Just beneath the lithosphere is a softer, relatively malleable layer of rock known as the asthenosphere. In more familiar terms, the Earth’s continents and many smaller plates form the lithosphere, while what they rest on forms the asthenosphere.

These rocky lithosphere plates slosh around the earth's service on their asthenosphere holding layer, colliding, rubbing and passing slowly above and below each other. During their motion, the plates sometimes become stuck at their edges like gears in a huge machine. Movement of the whole plate does not stop, and force builds up at the snag as the two bodies remain ensnared, until eventually they burst free releasing all of their energy. This energy travels rapidly outwards in great concentric circles, disrupting the landscape on the surfaces of the two plates as it goes. When a big enough snag occurs, the ensuing waves of force cause an earthquake.

Worth noting is that other sources of subterranean energy release, like magma movement beneath volcanic regions can also cause earthquakes. It is however the force from tectonic plate collisions and scrapes that generates the most of the earthquakes we experience. Curious readers can find out more in the following links.



Additional resources:

The Southern California Integrated GPS Network Education Module
http://scign.jpl.nasa.gov/learn/eq1.htm
This NASA affiliated site has great mechanical analysis as well as several illustrative flash animations.

US Geological Survey
http://earthquake.usgs.gov/learn/kids/eqscience.php
This US government site aims its language at younger learners, but should still be educational and engaging to an older audience.

The National Geographic
http://news.nationalgeographic.com/news/2009/02/photogalleries/humans-cause-earthquakes/index.html
Here you can find a photo-essay by national geographic on potential artificial causes of earthquakes.

Howstuffworks.com
http://science.howstuffworks.com/nature/natural-disasters/earthquake2.htm
Howstuffworks.com provides one of the more thorough web based coverages of earthquake terminology and theory.