An earthquake is the rapid relative displacement of the rock on either side of a fracture, or fault, in the interior of the solid earth. Elastic strain energy that had been stored in the rocks is suddenly released. Some of the energy released dissipates as friction along the fault. The rest is transferred to seismic waves that radiate outward in all directions from the initial point of rupture and cause ground motion at the earth's surface.

The severity of an earthquake can be measured in a variety of ways and places. These include measurements of what occurs on the surface, where ground motion can damage structures and infrastructure. Other measurements enable seismologists to infer what happened at the point within the earth where the rupture initiated, known as the hypocenter. The current practice among earth scientists and engineers is to use the word "magnitude" to characterize the energy released at an earthquake's hypocenter, and the word "intensity" to refer to the observed effects of an earthquake at the surface. While the magnitude of an earthquake is independent of the location at which measurements are made, intensity is dependent upon the distance from the rupture, the intervening geology, and local site conditions.

Most of the earth's seismic energy is released at the boundaries of tectonic plates. There are three types of plate boundaries. The first is known as the convergent type, where plates move toward one another. A dramatic example of a convergent plate boundary is the continent-continent collision zone between the Indian Subcontinent and Asia, which has resulted in the formation of the Himalayan Mountains. The most common manifestation of this type of boundary, however, is the subduction zone, where oceanic and continental crusts collide and the oceanic plate is thrust under the continental plate due to the oceanic plate's higher density. Most of the earth's seismic energy is released along these zones.

The second type of plate boundary is known as the transform, or strike-slip fault, where plates slide past one another. Perhaps the most well-known example of this type of plate boundary is the San Andreas fault in California.

The third type is known as the divergent plate boundary, where plates move away from one another. Examples are the Mid-Atlantic Ridge and the East-Pacific Rise. These plate boundaries occur almost exclusively deep in the ocean, and therefore present a negligible seismic hazard.

While the vast majority of earthquakes occur at plate boundaries, they can also occur in the interior of plates. Geologists believe that such areas are characterized by traces of ancient geological deformations or by variations in temperature and strength of the lithosphere. Earthquakes that occur in such areas are referred to as "intraplate" earthquakes. The New Madrid Seismic Zone in the Central United States is an example of region for which there is significant hazard from intraplate earthquakes.


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