EARTHQUAKE

any abrupt disturbance within the Earth that is tectonic or volcanic in origin and that results in the generation of elastic waves. The passage of such seismic waves through the Earth often causes violent shaking at its surface. The origin and distribution of most major earthquakes can be explained in terms of the plate tectonics theory. This theory postulates that the Earth's surface is made up of a number of large, rigid plates that move relative to one another and interact at their boundaries. The severest earthquakes tend to occur at convergent plate boundaries where one plate descends beneath the other. Most of these quakes originate more than 300 km (190 miles) below the surface and are associated with island arcs and trenches. Much seismicity also occurs near the margins where plates separate or slide past one another. Quakes at such sites tend to be of lower magnitude and are fairly shallow. In all such boundary regions, seismic waves are generated by the sudden fracturing of rock, which results when elastic strain accumulated during tectonic processes exceeds the strength of the rock. Three major zones of seismicity have been identified: (1) the circum-Pacific belt, which lies along plate margins around the Pacific Ocean and includes the well-known seismically active areas of Japan, Indonesia, New Guinea, the Andes Mountains, the western part of Central America, and the San Andreas Fault region of California, (2) the trans-Asiatic belt, extending from Mediterranean Europe eastward through Asia to the Pacific, and (3) the mid-ocean ridges, which form a connected worldwide rift system. Some earthquakes occur outside of these belts, away from plate boundaries. These intraplate earthquakes must be explained by mechanisms other than plate motions and suggest that stresses occasionally can exceed the strength of rock masses even within plates. Some phenomena akin to earthquakes, usually relatively minor, have been triggered by human activities that disturb the equilibrium of subsurface rock layerse.g., underground nuclear testing, impounding of water behind high dams, and the pumping of liquid wastes into the Earth through deep wells. The location of an earthquake is determined with a seismograph. This instrument records the oscillation of the ground caused by seismic waves that travel from their point of origin through the Earth or along its surface. A seismogram of a nearby earthquake is fairly simple, showing the arrival of P (or primary) waves, those that vibrate in the direction of propagation; slower-traveling S (or secondary) waves, those that vibrate at right angles to the direction of propagation; and surface waves, those of extremely high amplitude that skirt along the Earth's surface. In the case of distant earthquakes, the seismogram pattern tends to be more complex because it shows various types of seismic waves that originate from one point but are then reflected or refracted within the Earth's crust before reaching the seismograph. The relation between the arrival time of these waves and the epicentral distance (i.e., the distance from the point of origin) is expressed by a time-distance curve in which the arrival time is read on the vertical axis and the epicentral distance on the horizontal axis. If the arrival times of various seismic waves are read on the seismogram at a recording station and compared with standard time-distance curves, then the distance to the centre of an earthquake can be ascertained. The magnitude of an earthquake is usually expressed in terms of a logarithmic scale based on seismograph recordings of seismic-wave amplitudes. The numerical scale is so arranged that each increase in magnitude of one unit represents a 10-fold increase in earthquake sizei.e., an earthquake of magnitude 8 is 10,000 times as large as one of magnitude 4. Whereas the latter would be capable of causing only slight damage, the former constitutes a devastating seismic event. The scales that are commonly used are derivations of the Richter scale, which was introduced in southern California in 1935 and which, with successive refinements, held currency among seismologists for more than 40 years. The magnitude of an earthquake differs from its intensity, which is the perceptible degree of shaking of the Earth's surface and the attendant damage at any given location. In general, a quake's intensity decreases with distance from its epicentre, but other factors, including surface geology, may have a significant bearing on its effects on man-made structures. Large earthquakes have caused some of the worst disasters in history. No other natural phenomenon is as destructive over so large an area in so short a time. A major earthquake that struck Shensi province of China in 1556, for example, is estimated to have killed nearly 830,000 people while destroying entire towns and villages. The violent motions of the surface during large quakes can topple buildings. People are crushed and buried under the collapsing structures or are burned to death in ensuing building fires. Destructive, too, are the landslides and mudslides that may accompany an earthquake, as are tsunamis, the huge seismic sea waves induced by a disturbance in the adjacent seabed or by a submarine landslide triggered by an earthquake. Much research has been devoted to earthquake prediction since the mid-1960s, most notably by seismologists in China, Japan, Russia, and the United States. Various advances notwithstanding, no method has yet been devised to predict the time, place, or magnitude of earthquakes with a high degree of accuracy or consistency. Seismologists have found that major earthquakes are often preceded by certain measurable physical changes in the environment around their epicentres. These so-called precursor phenomena include the degree of crustal deformation in fault zones; occurrence of dilatancy (i.e., an increase in volume) of rocks; and a rise in radon concentrations in wells. Continual monitoring and close scrutiny of these and other related phenomena are expected to improve prediction capability in the future. any sudden disturbance within the Earth manifested at the surface by a shaking of the ground. This shaking, which accounts for the destructiveness of an earthquake, is caused by the passage of elastic waves through the Earth's rocks. These seismic waves are produced when some form of stored energy, such as elastic strain, chemical energy, or gravitational energy, is released suddenly. Few natural phenomena can wreak as much havoc as earthquakes. Over the centuries they have been responsible for millions of deaths and an incalculable amount of damage to property. While earthquakes have inspired dread and superstitious awe since ancient times, little was understood about them until the emergence of seismology at the beginning of the 20th century. Seismology, which involves the scientific study of all aspects of earthquakes, has yielded answers to such long-standing questions as why and how earthquakes occur. These matters are discussed in this article, as are the distribution, size, and effects of earthquakes. Additional reading The subject of earthquakes is dealt with mainly in books on seismology. Recommended elementary texts are Bruce A. Bolt, Earthquakes: A Primer (1978), and Inside the Earth: Evidence from Earthquakes (1982); and Charles F. Richter, Elementary Seismology (1958). Interesting discussions are also given by G.A. Eiby, Earthquakes (1967, reissued 1980); and Karl V. Steinbrugge, Earthquakes, Volcanoes, and Tsunamis: An Anatomy of Hazards (1982). More advanced texts that treat the theory of seismic waves in detail are K.E. Bullen and Bruce A. Bolt, An Introduction to the Theory of Seismology, 4th ed. (1985); and Keiiti Aki and Paul J. Richards, Quantitative Seismology: Theory and Methods, 2 vol. (1980). On the seismicity of the Earth, the most comprehensive treatment is still B. Gutenberg and Charles F. Richter, Seismicity of the Earth and Associated Phenomena, 2nd ed. (1954, reprinted 1965); see also J.P. Roth, The Seismicity of the Earth, 19531965 (1969). For a broad view of prediction, there is Tsuneji Rikitake, Earthquake Prediction (1976). On a history of discrimination between underground nuclear explosions and natural earthquakes, see Bruce A. Bolt, Nuclear Explosions and Earthquakes: The Parted Veil (1976). Bruce A. Bolt