XXXXThe Earth is formed of several layers. The outer layer consists of about a dozen large, odd shaped plates that slide around each other on top of the partly molten inner layer. Most earthquakes occur at the boundaries where the plates meet. Actually, the locations of earthquakes help scientists find the plate boundaries.

XXXXThere are three types of plate boundaries: spreading zones, transform faults, and subduction zones. At spreading zones, molten rock pushes two plates apart and adds new rock at their edges. Most spreading zones are found in oceans; for example, the North American and Eurasian plates are spreading apart along mid-Atlantic Ocean.

XXXX Spreading zones are shallow. Earthquakes occurring in this zone are located within 30 kilometers of the Earth's surface.

XXXXTransform faults are found where plates slide past one another. Earthquakes at transform faults tend to occur at shallow depths and form fairly straight lines of earthquakes.

xxxxSubduction zones are found where one plate pushes another downward into the mantle where it melts. Subduction zones are characterized by deep-ocean trenches, shallow to deep earthquakes, and mountain ranges with volcanoes. An earthquake is the vibration, sometimes violent, of the Earth's surface that follows a release of energy in the Earth's crust. This energy can be generated by a quick motion of plates by an explosion. Most destructive quakes are caused by dislocations of the crust. The crust may first bend and then, when the stress is too much, rocks break and "snap" to a new position. In the process of breaking "seismic waves" are generated. These waves travel outward from the source of the earthquake. Some of the vibrations are of high enough frequency to be heard, while others are of very low frequency. These vibrations cause the entire planet to ring like a bell.

XXXXA fault is a fracture in the Earth's crust along which two blocks of the crust have slipped. Normal faults occur in response to pulling or tension; the overlying block moves down the fault plane. Thrust faults occur in response to squeezing or compression; the overlying block moves up the dip of the fault plane. Lateral faults occur in response to either type of stress; the blocks move horizontally past one another. Geologists have found that earthquakes tend to reoccur along faults, which reflect areas of weakness in the Earth's crust. Even if a fault zone has recently experienced an earthquake, however, there is no guarantee that all the stress has been relieved. Another earthquake could still occur. Furthermore, relieving stress along one part of the fault may increase stress in another part; the New Madrid, Missouri earthquakes in January and February 1812 may have resulted from this phenomenon.

XXXXThe focal depth of an earthquake is the depth from the Earth's surface the focus. The focuses of most earthquakes are concentrated in the crust and upper mantle. The depth to the center of the Earth's core is about 3,960 miles, so even the deepest earthquakes originate in relatively shallow parts of the Earth's interior.

XXXXThe center of an earthquake is the point on the Earth's surface directly above the focus. The location of an earthquake is commonly described by the position of its center and by the depth of its focus.