by T Marie Allman

Created on: March 31, 2009

One of earth's most dangerous natural occurrences, earthquakes are a natural product of plate tectonics. The crust of the earth is not a solid mass of rock, but large pieces that affect each other through physical interaction. The movements of crustal rock cause the slow build-up of stress. The violent release of that stress causes an earthquake. Earthquakes' intensities range from barely perceptible to disastrous. They are the origin of phenomena such as tsunamis and crustal fissures. Aftereffects such as these can cause great damage and injury to people.

Following a world map that shows the earth's crustal plate boundaries and the frequency of earthquakes, one can see that earthquakes are most common along those boundaries. As continental and oceanic plates push against and subduct each other, adjacent rocks are bent and stressed. Large areas of these rocks can fracture along lines called faults. The two sides of a fault are pushed in opposite directions, causing them to slide past one another. When stress and pressure along a fault is released, the effect can be devastating. The most well known fault in the United States is the San Andreas Fault. The 1906 earthquake that nearly destroyed San Francisco was caused by slippage along the San Andreas when a two-hundred-seventy-mile section of land on the western side of the fault slid twenty-one feet northward.

When stress causes rock to fail, the energy released manifests as waves. Seismic (from Seismos, the Greek word for earthquake) waves follow the same behavior as water or light waves. There are two distinct types of waves produced by an earthquake, compressional and shear. In a compressional wave, rock particles expand and contract as the waves pass through them. The particles' line of movement matches that of the wave, making it move quickly. Compressional waves are known as primary, or P, waves because they travel faster through any medium and are the first to arrive at a seismograph station. In the second type of waves, shear waves, the rock particles move at right angles to the wave's line of direction. This type of movement is called transverse motion and causes the wave to move more slowly than a compressional wave. For this reason, shear waves are also called secondary, or S, waves. Both types of waves move along either a horizontal or vertical axis.

Because seismic waves share properties with other types of waves, they are useful in studying the interior of the earth. Seismic waves move at different

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