The earth's crust is made up of many different sections, known as plates. Deep under these plates is the mantle, which is always in motion (see diagram below). As the mantle circulates, pressure builds on the plates. In order to release the pressure, the plates need to move. Sometimes the movement is very slow, constantly releasing pressure, and sometimes the movement is very quick and violent from pressure build-up (an earthquake). Left: Interior of earth showing the cycling of the mantle. Notice the areas where plates are spreading, the mantle is pushing upward to form a ridge. Right: This is a map that identifies the major plates of the world by the different colors. Notice that where the Pacific Plate and the North American Plate meet in California is the San Andreas Fault line. There are four main types of plate boundaries (the area where two plates meet). They include: Transform boundaries: plates slide horizontally past each other, no crust is made or lost. (A) Divergent boundaries: two plates pull apart as new crust is made. (B) Convergent boundaries: two plates come together, with one plate diving under the other, destroying crust. (C) Plate boundary zones: broad areas in which boundaries are not well defined. This picture illustrates transform, divergent, and convergent boundaries. Notice the illustrations above the diagram representing the direction in which the plates are moving. (Cross section by José F. Vigil.)

Since there are three main types of plate boundaries, transform (sliding), divergent (extending), and convergent (compressing), there are three main types of earthquakes. Characteristics of Earthquakes   Transform Plate Boundary Divergent Plate Boundary Convergent Plate Boundary Location Close to surface Close to surface Any depth Intensity Ranging, usually less than 8.5 Usually not severe, less than 8.0 Can be very strong, some greater than 9.0 Learn more about the San Andreas fault in California!