seismology


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seismology

(sīzmŏl`əjē, sīs–), scientific study of earthquakesearthquake,
trembling or shaking movement of the earth's surface. Most earthquakes are minor tremors. Larger earthquakes usually begin with slight tremors but rapidly take the form of one or more violent shocks, and end in vibrations of gradually diminishing force called
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 and related phenomena, including the propagation of waves and shocks on or within the earth by natural or artificially generated seismic signals.

Seismographic Instruments

Instruments used to detect and record seismic disturbances are known as seismographs. Those in use today vary somewhat in design and function, but generally a heavy mass, either a pendulum or a large permanent magnet, is connected to a mechanical or optical recording device. When earthquake tremors occur, the pendulum or the magnet, because of inertia, remains still as the earth moves beneath, with the relative motion between the earth and the instrument magnified mainly by electrical amplifying apparatus. The graphic record, called the seismogram, can be used to establish information about an earthquake, e.g., its severity and distance. By using three instruments, each set to respond to motions from a different direction (north-south horizontal, east-west horizontal, and vertical), both the distance and the direction of the earth movement can be determined. Three or more widely spaced seismographic stations are required to pinpoint the location of earthquakes in remote regions.

Although seismographs have been used since their invention by John MilneMilne, John,
1850–1913, British seismologist, b. Liverpool, educated at King's College and the Royal School of Mines. He worked as a mining engineer in Newfoundland and Labrador and served (1874) as a geologist on a mining expedition to NW Arabia.
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 in 1880, until the end of the 20th cent. their placement was limited to land areas, creating conspicuous gaps in global seismic coverage under the oceans that cover most of the earth's surface. During the late 1990s geologists began to create an underwater network of geological observatories using undersea coaxial cables no longer used for communications. This enabled the more precise detection and measurement of seismic disturbances occurring between the continental land masses.

Development of Seismology

The American scientist John Winthrop (1714–79), often called the founder of seismology, was one of the first to make scientific studies of earthquakes. By analyzing seismic data from a 1909 earthquake near Zagreb (now in Croatia), the Austro-Hungarian meteorologist Andrija Mohorovičić discovered a boundary between the crust and mantle, now called the Mohorovičić discontinuity or Moho. Seismological studies were furthered by the U.S. seismologist Charles F. Richter, who invented the Richter scaleRichter scale
, measure of the magnitude of seismic waves from an earthquake. Devised in 1935 by the American seismologist Charles F. Richter (1900–1985) and technically known as the local magnitude scale, it has been superseded by the moment magnitude scale, which was
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 to determine an earthquake's magnitude. Each successive point on the logarithmic scale represents an increase by a factor of 10 in wave amplitude. A modified Mercalli scale, originally developed by the Italian seismologist Giuseppe Mercalli, is also based on the earthquake's effects on the surface.

Applications of Seismology

One aspect of seismology is concerned with measuring the speeds at which seismic waves travel through the earth. Past earthquake studies have shown that P, or primary/compressional, waves travel fastest through the earth; S, or secondary/transverse, waves cannot pass through liquids, allowing scientists to discern the earth's many boundary layers known as the crust, mantle, and core. For example, the disappearance of S waves below 1,800 mi (2,900 km) shows that the outer core of the earth is liquid. Seismologists also prepare seismic risk maps for earthquake-prone countries; these indicate the degree of seismic danger. In addition, seismologists use earthquake data to determine plate boundaries (see plate tectonicsplate tectonics,
theory that unifies many of the features and characteristics of continental drift and seafloor spreading into a coherent model and has revolutionized geologists' understanding of continents, ocean basins, mountains, and earth history.
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); active earthquake areas generally coincide with plate margins, both destructive and growing, and transform faultsfault,
in geology, fracture in the earth's crust in which the rock on one side of the fracture has measurable movement in relation to the rock on the other side. Faults on other planets and satellites of the solar system also have been recognized.
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.

An important commercial application of seismology is its use in prospecting for oil deposits. The first oil field to be discovered by this method was found in Texas in 1924. A portable seismograph is set up in the area to be investigated, and an explosive energy source is activated nearby; formerly, explosives such as dynamite were used to create the seismic waves, but they have been largely replaced by high-energy vibrators on land and air-gun arrays at sea. The waves generated are received by detectors known as geophones; on land, these are commonly placed in a fan-shaped pattern on the ground. From an interpretation of the waves created by the energy source and recorded by the seismograph, the detection of geological structures in which oil may be trapped is possible.

Seismic methods are sometimes used to locate subsurface water and to detect the underlying structure of the oceanic and continental crust. With the development of underground testing of nuclear devices, seismographic stations for their detection were set up throughout the world. Under the Comprehensive Test Ban Treaty (signed 1996 but not yet in force) an international monitoring system has been set up which includes many seismic stations; the detailed data collected is also used by contributing nations for purposes other than monitoring nuclear tests.

Bibliography

See B. F. Howell, An Introduction to Seismological Research: History and Development (1990); T. Lay and T. C. Wallace, eds., Modern Global Seismology (1995); H. A. Doyle, Seismology (1996). See also bibliography under earthquakeearthquake,
trembling or shaking movement of the earth's surface. Most earthquakes are minor tremors. Larger earthquakes usually begin with slight tremors but rapidly take the form of one or more violent shocks, and end in vibrations of gradually diminishing force called
..... Click the link for more information.
.

seismology

(sÿz-mol -ŏ-jee) The study of seismic waves, which propagate through the interior of the Earth, or other celestial bodies, such as the Moon, following earthquakes (or equivalent), explosions, or impact phenomena. There are two main types of wave: S-waves are shear waves that vibrate at right angles to their direction of motion; P-waves travel faster than S-waves and are compressional waves that oscillate along their direction of motion. The S- and P-waves travel deep into the body in question where they are refracted at the boundaries between layers of differing density. S-waves can travel only through a solid; P-waves can travel through solid, liquid, or gas. The waves are measured and analyzed to derive information about the events that generated them or about the internal structure of the body through which they travel. See also helioseismology.

Seismology

 

the branch of geophysics that studies earthquakes, including the causes and consequences, as well as measures to protect man-made structures. Seismic waves are the primary source of information. The interpretation of recordings of seismic waves makes it possible to study earthquakes, learn about the earth’s structure, find deposits of useful minerals, and determine the location of explosions, for example, nuclear explosions.

The prediction of earthquakes involves predicting the place, force, and time of occurrence. The problem of predicting the time and place of strong earthquakes is exceptionally difficult and has not yet been solved. The difficulty arises from the need to obtain information on processes occurring deep within the earth and the slow speed of differentiated tectonic movements that lead to earthquakes. Work in this direction involves the search for advanced indications of earthquakes. Such indications are phenomena caused by changes in the physicomechanical properties of the earth’s crust and mantle preceding an earthquake. They include variations over time in the propagation velocities of earthquake waves, a raising or lowering of the level of the ocean several hours before strong earthquakes, and a change in the electrical resistance of rocks.

Seismic zoning provides some degree of prediction by identifying the regions of potential maximum force and the average frequency of earthquakes. In order to do this, data from a network of seismological stations concerning the position of epicenters, depths of foci, magnitudes, and intensity of recorded earthquakes are analyzed and the correspondence of earthquakes to particular geological structures and regions of recent, intense tectonic movements is determined. Seismic observations are optimized by the rational selection of sites for seismological stations; proper site selection ensures good access to seismically active zones and minimizes seismic noise levels (microseisms).

Seismic zones are determined more precisely by seismic microzoning based on geological engineering surveying and seismometer observations. These studies provide the necessary data for earthquake-resistant construction and constitute the subject of engineering seismology.

An important problem in seismology is the obtaining of data whose interpretation will make possible a representation of the structure of the “solid” earth, that is, of the earth’s crust, mantle, and core. Seismic waves and changes in the velocities of such waves within the earth’s interior provide the basic material for this representation. Travel-time curves are used to solve this problem.

A new branch of seismology—the physics of earthquake foci—developed in the early 1970’s. This branch synthesizes data from seismology proper, theoretical mechanics, and the physics of rock disintegration. It studies the chief parameters of the focus: depth, dimensions, position of the rupture plane, seismic moment, and the characteristics of the processes of preparation, occurrence, and propagation of the rupture of rocks within the earth’s interior.

Modern seismology has available highly sensitive measuring equipment. The information received at seismological stations is processed by computers and automatic devices. A special branch of seismology called seismometry develops instruments and techniques for recording seismic oscillations.

Seismology arose in answer to the desire to explain the causes of destructive earthquakes and to find ways to build earthquake-resistant buildings. It developed as a separate science in the second half of the 19th century in connection with advances in geology and physics. In the late 19th century seismology began using instrument observations and physicomathematical methods of investigation. Among the scientists who made major contributions to the development of seismology were the Russian B. B. Golitsyn, the German geophysicist E. Wiechert (late 19th and early 20th centuries), B. Gutenberg, the British scientists J. Milne (second half of the 19th century) and H. Jeffreys, the Yugoslav geophysicist A. Mohorovičić, and the Japanese scientist F. Omori (early 20th century).

The Seismic Commission of the Russian Geographic Society was founded in Russia in 1888. The beginning of instrument seismology is linked with the establishment of the Permanent Central Seismic Commission of the St. Petersburg Academy of Sciences in 1900. In the USSR the primary research in seismology is carried on by the O. Iu. Shmidt Institute of Lithosphere Physics of the Academy of Sciences of the USSR (known as the Institute of Seismology of the Academy of Sciences of the USSR from 1928 to 1947 and as the Geophysical Institute from 1947 to 1956). Seismological institutions were established in the Union republics beginning in the 1930’s. In 1974 seismological research was being conducted by more than 30 specialized institutions and was being coordinated by the Interdepartmental Council on Seismology and Earthquake-resistant Construction (MSSSS) of the Presidium of the Academy of Sciences of the USSR.

International coordination in seismology is handled by the International Association of Seismology and Physics of the Earth’s Interior of the International Union of Geodesy and Geophysics. The chief periodical on seismology in the USSR is Izvestiia AN SSSR: Seriia geofizicheskaia (Proceedings of the Academy of Sciences of the USSR: Geophysical Series; since 1965, under the title Seriia fiziki Zemli [Physics of the Earth Series]). The major foreign periodicals are the Bulletin of the Seismological Society of America (Stanford, since 1911), Bulletin of the Earthquake Research Institute, Tokyo University (Tokyo, since 1926), Journal of Physics of the Earth (Tokyo, since 1952), and Geophysical Journal: Royal Astronomical Society (London, since 1958).

REFERENCES

Savarenskii, E. F., and D. P. Kirnos. Elementy seismologii i seismometrii, 2nd ed. Moscow, 1955.
Golitsyn, B. B. Izbr. trudy, vols. 1–2. Moscow, 1960.
Atlas zemletriasenii v SSSR. Moscow, 1962.
Medvedev, S. V. Inzhenernaia seismologiia. Moscow, 1962.
Predskazanie zemletriasenii: Sb. Moscow, 1968. (Translated from English.)
Seismicheskoe raionirovanie SSSR: Sb. Moscow, 1968.
Eksperimental’naia seismologiia: Sb. st. Moscow, 1971.
Savarenskii, E. F. Seismicheskie volny. Moscow, 1972.
Poiski predvestnikov zemletriasenii na prognosticheskikh poligonakh [collection of articles]. Moscow, 1974.
Fizika ochaga zemletriasenii. Moscow, 1975.

E. F. SAVARENSKII

seismology

[sīz′mäl·ə·jē]
(geophysics)
The study of earthquakes.
The science of strain-wave propagation in the earth.

seismology

the branch of geology concerned with the study of earthquakes and seismic waves
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