The Physics Of An Earthquake

Earthquakes are vibrations produced in the earth's outer layer, or crust, when forces pushing on a mass of rock overcome the friction holding the rock in place and blocks of rock slip against each other. The vibrations can range from barely noticeable to verry destructive. There are six types of shock waves. Two are classified as body waves which means they travel through the earth's interior and the other four are surface waves. The waves are changed by the rock types or formations they hit. Primary or compressional waves (P waves) send particles moveing back and forth in the same direction as the waves are traveling, secondary or transverse shear waves (S waves) send vibrations perpendicular to their direction of travel. P waves always travel at higher velocities than S waves. Three general classes of earthquakes are now recognized as tectonic, volcanic, and artificially produced. The tectonic variety is by far the most destructive. The most commen cause of tectonic quakes is stresses by movements of the dozen of major and minor plates that make up the earth's crust . Most tectonic quakes occur at the boundaries of these plates, in zones where one plate slides past another

Subduction-zone quakes account for nearly half of the world's destructive seismic events and 75 percent of the earth's seismic energy. They are along the so-called Ring of Fire, a narrow band about 38,600 km long, that coincides with the sides of the Pacific Ocean. The points at which crustal rupture occurs in such quakes tend to be far below the earth's surface, at depths of up to 645 km.

Not all subduction zones are subject to frequent earthquakes.

The frequency and magnitude of earthquakes around subduction zones are related to the direction in which the plates are moving. If two plates moving in the same general direction come close together, generally the edge of one plate will slide below the other at a sharp angle. This reduces the amount of area in which the plates touch, so the subduction zone does not produce many earthquakes and any earthquakes it does produce are not as strong. If two plates are sliding beside each other, one plate will often be forced under the other at a shallow angle, making a large area of friction. This produces more frequent, stronger earthquakes.

Tectonic earthquakes beyond the Ring of Fire occur in a variety of geological settings.