An earthquake is a trembling or shaking of the ground that is caused by a release of energy stored in rocks beneath the surface of the earth.. There are great forces that take place beneath the surface of the earth that put a stress on these rocks, which bend or change the volume in the rocks. This sudden release of energy when the rocks break causes one huge mass if rock to slide past another mass of rock in a different relative position. The crack between these two masses of rock is called the fault. A theory called the elastic rebound theory explains why an earthquake takes place. This theory basically states the a release of the stored strain in rocks cause movement along the fault. From there, tectonic forces act on a mass of rocks as they move pass each other. This makes the bending of the rock more severe. The rocks move pass each other on the fault, either vertically or horizontally, causing a breaking of the strength of the rock, triggering an earthquake.
 
 

The center of an earthquake contains a focus and epicenter. The focus of an earthquake is the point where rocks first break along a fault. The epicenter is the point on the earth's surface directly above the focus. These two parts of an earthquake are located where seismic waves are originated which is produced every time an earthquake takes place. With this come the P wave and the S wave. The P wave is longitudinal wave which vibrates parallel to the direction as the waves are moving. The P wave is very fast, so it comes before the S wave. The S , or secondary wave, is a slower wave. This wave is progated by motion, much like sharing and stretching a rope. The rock vibrates perpendicular to the direction of wave goes. These two waves both move through solid rock. The center of seismic waves is the point of initial movement on a fault. This rupture begins at the focus and spreads rapidly while it decreases rock displacement from the focus.Two other types of seismic waves are body waves , which travel through the earths interior while spreading outward from the focus in all directions, and surface waves, which travel on the earth's surface away from the epicenter. The rock movement die out with depth into the earth.
 
 

In order to locate some of the earth's resources, educing earthquakes can be helpful. Vibrasets on truck, which beat at the earth's surface are used to generate waves like that in the earth to determine where rocks are in earthquakes and what types of oils and or gases that are present in the surfaces of the earth.
 
 

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