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Earthquake, any sudden shaking of the ground caused by the passage of seismic waves through Earth’s rocks. Earthquakes occur most often along geologic faults, narrow zones where rock masses move in relation to one another.
Seismic wave, vibration generated by an earthquake, explosion, or similar energetic source and propagated within the Earth or along its surface. Earthquakes generate four principal types of elastic waves; two, known as body waves, travel within the Earth, whereas the other two, called surface.
Earthquake - Magnitude, Seismology, Epicenter: Earthquake magnitude is a measure of the “size,” or amplitude, of the seismic waves generated by an earthquake source and recorded by seismographs. (The types and nature of these waves are described in the section Seismic waves.)
Epicentre, point on the surface of the Earth that is directly above the underground point (called the focus) where fault rupture commences, producing an earthquake. The effects of the earthquake may not be most severe in the vicinity of the epicentre. The epicentre can be located by computing arcs.
Earthquake - Magnitude, Intensity, Effects: The violence of seismic shaking varies considerably over a single affected area. Because the entire range of observed effects is not capable of simple quantitative definition, the strength of the shaking is commonly estimated by reference to intensity scales that describe the effects in qualitative terms.
In an earthquake, huge masses of rock move beneath Earth’s surface and cause the ground to shake. Earthquakes occur constantly around the world. Often they are too small for people to feel at all, but some earthquakes are massive. They can cause great destruction.
Earthquake - Tectonics, Seismology, Faults: Tectonic earthquakes are explained by the so-called elastic rebound theory, formulated by the American geologist Harry Fielding Reid after the San Andreas Fault ruptured in 1906, generating the great San Francisco earthquake.
Tsunami, catastrophic ocean wave usually caused by a submarine earthquake, by a landslide, or by a volcanic eruption. In deep water it travels as fast as 800 km (500 miles) per hour, with enormous wavelengths of more than 500 km (310 miles) but small wave amplitudes of about 30 to 60 cm (1 to 2 feet).
Richter scale, widely used quantitative measure of an earthquake’s magnitude (size), devised in 1935 by American seismologists Charles F. Richter and Beno Gutenberg. Magnitude is determined using the logarithm of the amplitude (height) of the largest seismic wave calibrated to a scale by a seismograph.
Seismology, scientific discipline that is concerned with the study of earthquakes and of the propagation of seismic waves within the Earth. A branch of geophysics, it has provided much information about the composition and state of the planet’s interior.