An earthquake occurs when tectonic pressure builds up along a fault. After time enough pressure builds up and causes the plates to move past each other. The point of movement in which this energy is released is called the epicenter of the earthquake. This energy is released in waves known as P and S waves. The P wave travels faster than the S wave, therefore if the speed of the P and S waves are known, the distance of the epicenter can be measured. The distance of the epicenter is measured by the time span between the P and S waves (T2 - T1 = distance, where T1 = time of the P wave, T2 = time of the S wave) . A location of an epicenter can be determined by the use of multiple P and S wave readings. For instance, if one wave reading location reported a distance from the epicenter of 100 miles, a second wave reading reported an epicenter 200 miles away, and a third location reported an epicenter of 150 miles away, then if you draw an imaginary radius around each of the locations of their respective readings the point at which these radii cross will be the position of the epicenter.
 
 

Mankind can use these waves for other purposes. In Oil and Gas exploration a vibrasies or thumper truck send artificial S and P waves through the earth. These waves reflect off different formations at different angles and rates. As the waves pass though different formations the waves are bent depending on fluids or densities of the formation. As the wave bends their reflective angle is changed. Sensors at the surface pick up and record the differences in these reflected waves. A geologist can then interpret this information to determine types and economic value of the oil and gas resiviors.These waves can also be used in much the same way to determine faults and folds in the earths crust.

Nuclear bomb testing can also be tracked by this method. When a nuclear bomb is detonated it releases enough energy to create S and P waves. Seismographs throughout the world can pick up on these waves and therefore be able to alert neighboring countries of hostile activity.
 
 

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