Shaking and vibration at the surface of the earth resulting from underground movement along a fault plane of from volcanic activity; SYN. quake, temblor, seism.
Shaking of the Earth’s surface as a result of the sudden release of stresses built up in the Earth’s crust. The study of earthquakes is called seismology. Most earthquakes occur along faults (fractures or breaks) in the crust. Plate tectonic movements generate the major proportion: as two plates move past each other they can become jammed and deformed, and a series of shock waves (seismic waves) occur when they spring free. Their force (magnitude) is measured on the Richter scale, and their effect (intensity) on the Mercalli scale. The point at which an earthquake originates is the seismic focus; the point on the Earth’s surface directly above this is the epicenter.
In 1987 a California earthquake was successfully predicted by measurement of underground pressure waves; prediction attempts have also involved the study of such phenomena as the change in gases issuing from the crust, the level of water in wells, slight deformation of the rock surface, a sequence of minor tremors, and the behavior of animals. The possibility of earthquake prevention is remote. However, rock slippage might be slowed at movement points or promoted at stoppage points by the extraction or injection of large quantities of water underground, since water serves as a lubricant. This would ease overall pressure.
Most earthquakes happen at sea and cause little damage. However, when severe earthquakes occur in highly populated areas they can cause great destruction and loss of life. A reliable form of earthquake prediction has yet to be developed, although the seismic gap theory has had some success in identifying likely locations.
The San Andreas fault in California, where the North American and Pacific plates move past each other, is the site of many earthquakes. It is part of the “ring of fire”, the belt of earthquakes and volcanoes circling the Pacific.