Earthquake, vibration or series of vibrations in the earth's crust. Earthquakes are the result of sudden vertical or horizontal movements along faults, or fractures, in the earth's crust. Some faults, such as the 600-mi (966-km) San Andreas fault in California, can be seen on the earth's surface, but most of the faults associated with earthquakes are underground. Scientists have recently suggested that the origin of many earthquakes is linked with continental drift. Many earthquakes also occur under the oceans. Underwater earthquakes or earthquakes that occur near coastlines may cause destructive waves called tsunamis, which may travel vast distances at speeds approaching 500 mph (805 kmph). Seismologists have studied earthquakes by recording the seismic waves that travel through the earth. Information from several seismographic stations makes it possible to locate the point of origin, or focus, of an earthquake, and also the epicenter, the point on the earth's surface directly above the focus. The most common and most destructive earthquakes are shallow-focus, that is, their focus lies within about 30 mi (48 km) beneath the epicenter. Intermediate and deep-focus earthquakes, which may occur as deep as 400 mi (644 km) below the surface, are less destructive. The magnitude of an earthquake is a measure of the strength of the seismic wave it generates. It is usually measured on the Richter scale, devised by U.S. seismologist C.F. Richter in 1935. The scale ranges from 0 to 8.4, with higher numbers signifying greater magnitude. The intensity of an earthquake is a measure of its effect in a particular area and varies with the distance from the epicenter. The Modified Mescalli scale, which ranges from 1 (not felt) to 12 (nearly total damage), is normally used. Although scientific knowledge about earthquakes has greatly increased, there is still no effective way of forecasting them.
See also: Seismology.