Seismology is the scientific study of earthquakes and the propagation of elastic waves through the Earth. Earthquakes occure when a large amount of energy that is built up over time suddenly get released. The Earth's crust and upper mantle are made up of many plates that are in slow, but constant motion. Most of the time there is little to no motion at plate boundaries, so forces build as plates try to slide by each other. Occasionally, the stress becomes to great, and large fault zones at the plate boundaries cause sudden and violent movements. There are three major types of faults, 1) strike-slip, 2) normal, and 3) reverse (shown to the right).