Depending on how strong they are and where they strike, earthquakes can be some of the worst natural disasters, taking thousands of lives and creating billions of dollars of damage. Earthquakes are the result of the sudden movement of rock along a fault zone beneath the surface, usually centered in tectonically active areas. An earthquake beneath the ocean results in huge waves called seismic sea waves (tsunamis) that devastate coastlines. Scientists who study earthquakes
(seismologists) hope to eventually predict earthquakes before they strike. Engineers are also challenged to develop new designs that will enable buildings to withstand the forces of earthquakes.
An earthquake is the shaking of the earth caused by the sudden release of energy from rocks under tectonic stress. Most earthquakes are associated with rock movements along faults below the surface of the earth. Because of friction and high confining pressure the fault blocks don't move until the tectonic stress becomes great enough to overcome the frictional force. Earthquakes may also result according to the elastic rebound theory, which suggests that in some cases energy is stored in rock that is being bent (deformed) by tectonic forces until the energy in the rock exceeds the bonding strengths between minerals, and the rock breaks. When the rock breaks, it suddenly returns to its predeformed shape, and the crust moves violently as a result of the quickly released force. This results in the formation of a new fault. As an analogy, you might think of a flexible plastic rod being bent until it reaches its breaking point. After it breaks, its two ends spring back from their curve into a straight line, the energy that curved them having been released. The displacement along the fault zone typically ranges from about 1 to 7 meters; earthquakes rarely last longer than a few minutes.
Not all earthquakes are associated with existing faults—some are related to deeply buried fold structures, thrust faults, and volcanic environments in which magma is forcing its way to the surface. Earthquakes can also create new faults. Some geologists also theorize that the deeper earthquakes are a result of mineral transformations in cold subducted plates that enter the hotter mantle.