IV.

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.