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 very destructive. There are 6 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 which are referred to as "P waves" send particles moving back and forth in the same direction as the waves are traveling, secondary or transverse shear waves which are known as "S waves" send vibrations perpendicular to their direction of travel. P ...view middle of the document...
They are along the Ring of Fire, a narrow band 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. Midocean ridges the seafloor spreading centers of plate tectonics are the sites of numerous such events of moderate intensity that take place at relatively shallow depths. One other category of tectonic earthquake includes the infrequent but large and destructive quakes that occur in areas far removed from other forms of tectonic activity. Earthquakes originate as magma, molten rock from a layer in the earth's mantle called the asthenosphere, works its way upward, filling the chambers beneath a volcano. As the flanks and summit of the volcano swell and are tilted, rupture of the strained rocks may be signaled by many small earthquakes. Another effect of earthquakes is the generation, usually by subsea tremors, of so-called tidal waves. Because such waves are not related to the tides, they are more properly called seismic sea waves or tsunamis. These big waves of water have struck coastlines with such violent strength that strike towns that have been destroyed by them unpredictably.