SEISMOLOGY

scientific discipline that is concerned with the study of earthquakes and of the propagation of seismic (elastic) waves within the Earth. A branch of geophysics (q.v.), it has provided much information about the composition and state of the planet's interior. The goals of seismological investigations may be local or regional, as in the attempt to determine subsurface faults and other structures in petroleum or mineral exploration, or they may be of global significance, as in attempts to determine phase discontinuities in the Earth's interior, the geophysical characteristics of island arcs, oceanic trenches, or mid-oceanic ridges, or the elastic properties of Earth materials generally. In recent years, increasing attention has been devoted to earthquake prediction and to assessing seismic risks at different geographic sites in an effort to reduce the hazards of earthquakes. Moreover, seismologists have studied quakes induced by human activities, such as impounding water behind high dams, injecting fluids into deep wells, and detonating underground nuclear explosions. The objective of this research is to find ways of controlling natural earthquakes.