Earthquakes occur as a result of movement deep in the Earth's crust, often resulting in faults. These faults or cracks are created by the slow drift of the Earth's major plates. The most commonly occuring and most destructive earthquakes occur at a very shallow point below the earth's surface, usually about thirty miles beneath the surface. Less destructive earthquakes can occur up to 400 miles below the surface. Earthquakes can also take place under water and this event occasionaly produces giant tidal waves called tsunamis. Historically the most seismic activity has been along the border area of two or more plates, although some on occasion do occur away from a plate boundary.

The magnitude of an earthquake is most commonly measured using the Richter scale, a logarithmic measure of the amplitude recorded by a seismograph. This scale ranges from 0 to greater than 9, with 0 being the lowest magnitude and least intense, and 8.6 being the highest magnitude recorded. Another device used to measure earthquakes is the Mercalli scale, which is a scale that ranges from 1 to 12, with 12 being the most intense. Categories are established based on the type of damage recorded.
 

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