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An earthquake is the vibration felt when energy is released by the sudden displacement of rock along faults, or large fractures, within the Earth's Crust.

Most earthquakes can be explained by plate tectonics and the elastic rebound theory. The theory was proposed by H.F. Reid of Johns Hopkins University in 1906. It says that the constant motion of rocks along one side of a fault boundary causes the rocks on the opposite side to bend.

The bending leads to a build up of elastic energy. Eventually the frictional forces holding the rocks together are overcome and the rocks break at the weakest point along the fault line, known as the focus. The deformed rock experiences slippage and then snaps back to its original position. The energy released by this slippage causes earthquake vibrations.

The released energy travels through the Earth in the form of waves. These waves are separated into three classes: primary, secondary and surface waves.

Primary, or P waves, are the first waves felt because they are the fastest. They move in a compressional, "push-pull" manner similar to a spring that temporarily changes the volume of the material they're moving through. These waves can travel through liquid, solid and gaseous matter.

Secondary, or S waves, are felt next. These waves move in an oscillatory, "up and down" manner similar to shaking a rope that temporarily changes the shape of the material they're traveling through. Because liquids respond to changes in volume but not shape, they will not transmit S waves.

Finally the surface waves are felt. These waves are more complex. Rayleigh waves move up and down like ocean swells along the Earth's surface while Love waves move side to side or horizontally. Surface waves are most damaging to man-made structures.

Because seismic waves travel in all directions from the focus, they can be recorded from any point on the earth's surface. Locations of earthquakes can be established by finding the epicenter, the point on the Earth's surface directly above the focus, by using a method called triangulation. Records of the types of waves that reach certain areas of the Earth have provided clues about the physical state of the Earth's interior.

By improving our understanding of the Earth and its forces, scientists can improve the stability of building structures near seismically active zones to save lives and lessen damages and maybe someday even predict earthquakes.

The Crust
The Earth is composed of three main layers of rock: the core, the mantle and the crust. The core is the center of the Earth and is separated into a solid, metallic inner core and a molten outer core with an overall thickness of about 2161 km.

Plate Tectonics
The theory of plate tectonics was proposed in 1968 to explain why similar fossils and rock structures are found in landmasses now separated by vast oceans. It says that the earth is divided into about 20 rigid plates that are in continuous motion in relation to one another.

It is the movement of these plates that creates stresses in the rock that result in earthquakes, most of which occur along plate boundaries.

Elastic energy
Energy created when a rock is deformed elastically, like a stretched rubber band. Rocks can also be deformed viscously, like silly putty, or in a combination of the two.

A mathematical method for locating the epicenter of an earthquake.