seismology , scientific study of earthquakes 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 Milne 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 scale 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 tectonics ); active earthquake areas generally coincide with plate margins, both destructive and growing, and transform faults .

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 earthquake.

Seismology

Seismology is the science that studies earthquakes and phenomena connected with them. Seismology is a branch of geophysics.

Seismology attempts to explain the origin of earthquakes, where, when, and why they occur, what accompanies them, and how to forecast them. Earthquakes were mentioned in written historical documents as early several thousands of years ago, but their serious study began only in the nineteenth century. As a rough guide, the earthquake is a vibration of the ground tangible in a definite place; the stronger these vibrations are, the more damage an earthquake can cause.

Two variables are usually used for describing the earthquake power: magnitude and intensity. Magnitude is an objective parameter, which is connected with the ground displacement at the point of its measurement; the bigger the displacement, the stronger is the earthquake. Earthquakes with magnitudes bigger than 5–6 are considered powerful ones. Intensity is a parameter that is not measured by a device. Different factors are taken into account to determine the intensity of an earthquake, and its value varies relative to locations accessed. The Modified Mercalli Intensity Scale determines earthquake intensity in the United States by gathering information, including witness accounts and building damage, and assigning a numeral scale from I (low intensity, no earth movement felt) to XII (visible earth movement seen, severe building damage).

The earth's crust , the upper layer of Earth's surface, consists of a solid medium with different values of parameters in different points, and is exposed to permanent action of different forces, which are also irregular. Action of these forces can lead to a situation in which some parts of the crust can occur under the condition of very high tension (for example, like a rod that is curved). If the tension remains too high, the crust is damaged in some points (the rod is broken). In this case, a very big amount of energy becomes free, and this energy transfers into elastic waves of different kinds, which can spread to great distances from the damage point. This illustrates a simplified model of an earthquake.

It is possible to distinguish the areas where earthquakes occur more often than in other places. Usually, these are mountain areas, and areas circumambient the Pacific (Japan, for example). Seismology studies these areas together with geology , in which during the late nineteenth century arose the special theory (plate tectonics ) which explains a set of phenomena, in particular the occurrence of active seismic zones and non-active zones (platform regions). Earthquakes are practically nonexistent in nonactive zones (for example, the Russian platform can be referred to as one of these regions).

Both seismology and geology use the achievements of physics (such as elasticity theory and hydrodynamics) in the creation of theories such as plate tectonics. In studying earthquakes, the oscillation theory and the theory of wave propagation in elastic media are used. The main devices used for earthquake study are seismometers, which record media oscillations at the point of the device location. Nowadays, such devices are located in many points on the earth's surface, on the ocean bottom, in shafts—often they are joined in special nets. The analysis of seismometers (seismograms), which take into account wave propagation theory, performs another important role; it permits scientists to understand deep-Earth structure. Even with improved theory, calculation methods, and equipment used in seismology, reliable forecasts of earthquakes still cannot be achieved.

Another important aspect of seismology is that its applied methods permit scientists to search useful minerals , especially oil. Seismologic methods also give the most precise results in underground nuclear tests control.

See also Earth, interior structure; Faults and fractures; Mid-ocean ridges and rifts; Mid-plate earthquakes; Petroleum detection; Richter scale; Subduction zone

seismic tomography Seismic tomography is a relatively new method of determining the velocity structure of the Earth. In this method, an image of an object (a heterogeneity), is formed by analysing the travel times of many waves that have passed through the target area for which the image is sought. Some reference Earth model (a velocity structure) is assumed; variations from this model are determined and are typically expressed as percentage of changes in velocity. This approach is similar to that used in medicine to form images from X-rays or ultrasound such as the CAT scan (computerized axial tomography) and is fundamentally different from the traditional approaches used in seismic reflection and refraction studies. Unfortunately, tomographic studies of the Earth employ seismic waves which bend as they travel, and the distribution of seismographs and seismic sources is usually less than ideal. For example, earthquakes are concentrated in distinct regions in the Earth and few seismographs operate in oceanic areas. However, recent advances have lead to the installation of many new, high-quality seismography stations and the ability to process large quantities of data. In addition, a particular region can be targeted by installing portable seismographs for a limited period of time. Also, as in other geophysical techniques, most developments can be applied on either a local or global scale. Thus, seismic tomography is an active area of research that is producing interesting new images of the velocity variations in the Earth.

The basic concept behind seismic tomography is that the travel time of a body wave travelling through the Earth can be expressed by an integral equation which in effect states that the time of travel is equal to the distance divided by the velocity. If the travel time for an individual seismic wave is different from what we expect from the reference Earth model, we say that there is a travel-time residual. At this stage we would know only that there was a velocity anomaly somewhere along the ray path that the wave had travelled. The essence of seismic tomography is to employ a large number of such observations for waves that have travelled by a variety of ray paths and to locate the position of the velocity anomaly. There are many mathematical approaches to this complex problem, but the most commonly employed one that is to approximate the Earth by a 3-dimensional mesh of blocks. The travel-time integral in the equation then becomes the summation of the travel times through each block intersected by the ray path. Many observations lead to a large system of equations in which the unknowns are the velocities of the blocks. The quality of the solution depends on the number of observations and their accuracy, the distribution of ray paths, and the degree to which the block model actually approximates to the true velocity structure.

A typical example is shown in Fig. 1a. This is from a study conducted in a volcanic field south of the Grand Canyon in Arizona. The velocity variations are shown by changes of tone; dark tones represent high velocities. A zone of high velocity was detected under the volcanic centre in the field. The ray-path coverage (Fig. 1b) must be shown because blocks that are not sampled obviously do not have their velocity determined.

Other approaches to this problem are being investigated. In particular, spherical harmonic expansions have been used to look at global-scale velocity variations. A great deal of research is also being undertaken to study more than just travel-time pertubations and isotropic velocity structures (see seismic anisotropy).

G. R. Keller

Bibliography

Iyer, H. M. (1989) Seismic tomography. In James, D. (ed.) The encyclopedia of solid Earth geophysics, pp. 1133–51. Van Nostrand-Reinhold, New York.
Lay, T. and and Wallace, T. C. (1995) Modern global seismology. Academic Press, San Diego.
Nolet, G. (ed.) (1987) Seismic tomography. D. Reidel Publishing Co., Dordrecht.

seismic stratigraphy (seismostratigraphy) The study and interpretation of information obtained by seismic-reflection profiling in order to construct subsurface stratigraphic cross-sections. Analysis of seismic reflections on a seismic section (see SEISMIC RECORD) can identify buried stratal surfaces that, when traced laterally and continuously, represent surfaces of synchronous deposition or their correlative unconformity surfaces. The character of a reflection may vary as the seismic profile moves across a facies boundary, but the continued presence of the reflection is of chronostratigraphic significance. More detailed information regarding the age and lithology of the subsurface strata may be gathered by means of geophysical well logging. See also CHRONOSTRATIGRAPHIC CORRELATION CHART; and DEPOSITIONAL SEQUENCE.

seismic tomography A range of methods in which the subsurface is divided into a box-grid whose elements are illuminated by seismic rays and the physical character of each element computed. The results are displayed as colour-coded contour maps of subsurface planes. In borehole tomography, two holes are used; a source is moved up one while detectors record along the length of the other. By using a succession of shot positions, most of the box elements in the plane linking the two boreholes are illuminated satisfactorily. Seismic tomography is used to study geologic structures in detail. It has been extended to investigate the mantle and, increasingly, the effectiveness of extraction techniques in hydrocarbon reservoirs.

seismology The study of elastic (seismic) waves and how they are produced. Global seismology is the study of seismic waves from earthquakes (and to a lesser extent nuclear explosions), to investigate the structure of and processes within the Earth. In exploration seismology, artificially generated seismic waves are used in the search for resources (e.g. hydrocarbons, etc.) and the study of the Earth's surface and near-surface. Planetary seismology is the use of seismic waves to investigate the structure of and processes within planets and natural satellites in the solar system.

seis·mol·o·gy / sīzˈmäləjē/ • n. the branch of science concerned with earthquakes and related phenomena. DERIVATIVES: seis·mo·log·i·cal / ˌsīzməˈläjikəl/ adj. seis·mo·log·i·cal·ly / ˌsīzməˈläjik(ə)lē/ adv. seis·mol·o·gist / -jist/ n.

seismology Study of seismic waves, the shock waves produced by earthquakes. The velocity of seismic waves varies according to the material through which they pass. Primary (P) and secondary (S) waves are transmitted by the solid earth. Only P waves are transmitted through fluid zones. Seismic waves are detected and recorded by seismographs.

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