Seismology, basically, the science of earthquakes, involving observations of natural ground vibrations and artificially generated seismic signals, with many theoretical and practical ramifications (see Earthquake). A branch of geophysics, seismology has made vital contributions to understanding the structure of the earth's interior.

Seismic Phenomena

 

Different kinds of seismic waves are produced by the deformation of rock materials. A sudden slip along a fault, for example, produces both longitudinal push-pull (P) and transverse shear (S) waves. Compressional trains of P waves, set up by an abrupt push (or pull) in the direction of wave propagation, cause surface formations to shake back and forth. Sudden shear displacements move through materials with slower S-wave velocity as vertical planes shake up and down.

When P and S waves encounter a boundary such as Mohorovicic discontinuity (Moho), which lies between the crust and the mantle, they are partly reflected, refracted, and transmitted, breaking up into several other types of waves as they pass through the earth. Travel times depend on compressional and S-wave velocity changes as they pass through materials with different elastic properties. Crustal granitic rocks typically show P-wave velocities of 6 km/sec (3.6 mi/sec), whereas underlying mafic and ultramafic rocks (dark rocks containing increasing amounts of magnesium and iron) show velocities of 7 and 8 km/sec (4.2 and 4.8 mi/sec), respectively. In addition to P and S waves—body-wave types—two surface seismic waves are the Love waves, named for the British geophysicist Augustus E. H. Love, and Rayleigh waves, named after the British physicist John Rayleigh. These waves travel fast and are guided in their propagation by the earth's surface.

Means of Study

Longitudinal, transverse, and surface seismic waves cause vibrations at points where they reach the earth's surface. Seismic instruments have been designed to detect these movements through electromagnetic or optical methods. The main instruments, called seismographs, were perfected following the development by the German scientist Emil Wiechert of a horizontal seismograph about the turn of the century.

Some instruments, such as the electromagnetic pendulum seismometer, employ electromagnetic recording; that is, induced tension passes through an electric amplifier to a galvanometer. A photographic recorder scans a rapidly moving film, making sensitive time-movement registrations. Refraction and reflection waves are usually recorded on magnetic tapes, which are readily adapted to computer analysis. Strain seismographs, employing electronic measurement of the change in distance between two concrete pylons about 30 m (about 100 ft) apart, can detect compressional and extensional responses in the ground during seismic vibrations. The Benioff linear strain seismograph detects strains related to tectonic processes, those associated with propagating seismic waves, and tidal yielding of the solid earth. Still more recent inventions used in seismology include rotation seismographs; tiltmeters; wide-frequency-band, long-period seismographs; and ocean-bottom seismographs.

Similar seismographs are deployed at stations around the world to record signals from earthquakes and underground nuclear explosions. The World Wide Standard Seismograph Network (WWSSN) incorporates some 125 stations.

Applications

 

Basic seismological research concentrates on better understanding the origin and propagation of earthquakes and the internal structure of the earth. According to the elastic rebound theory, strain that has built up over many years is suddenly released by fault movements as intense seismic vibrations.

Strong tremors can reduce structural edifices to rubble in seconds; geologists and engineers therefore consider a variety of quake-related factors in building design, because dams, nuclear power plants, waste disposal sites, roads, missile silos, buildings, and other structures that are constructed in seismogenic provinces must be able to withstand specified ground motion.

Seismic prospecting methods initiate artificial seismic waves at a given point by such means as explosives; at other points, using geophones and other apparatus, they determine the time of arrival of the energy that is refracted or reflected by discontinuities in rock formations. These techniques produce seismic refraction or seismic reflection profiles, depending on which of the two kinds of phenomena is being recorded. In seismic exploration for petroleum, advanced signal-generating techniques are combined with sophisticated magnetic-tape and digital-recording systems for enhanced data analysis.

Seismic reflection profiling, developed in the 1940s as a petroleum-exploration technique, has been used in recent years to conduct basic research. In an unprecedented program to decipher the structure of the hidden continental crust, COCORP (Consortium for Continental Reflection Profiling) has used this technique to probe rock tens of kilometers deep, thereby resolving many of the enigmas of the origin and history of the crust of North America. Among COCORP's major discoveries was a nearly horizontal fault (see Fault) with over 200 km (125 mi) of displacement. This structure, in the southern Appalachians of Georgia and South Carolina, represents the surface along which a great sheet of crystalline rock was thrust up over sedimentary rocks as a result of the collision between North America and Africa during the Permian Period, 250 million years ago. See Plate Tectonics.

Investigations conducted in the North Sea, north of Scotland, by a British group called BIRPS (British Institutions Reflection Profiling Syndicate), have delineated even deeper structures, some extending below the crust into the earth's mantle, almost 110 km (70 mi) deep.

 

Contributed by:

Charles W. Finkl, Jr.