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The approach to understanding the physics of an earthquake is still largely empirical. Representations of the focal mechanism by a system of forces acting at a point, by a moving point source, and by distributions of elastic dislocations have contributed to an interpretation of the distribution in space and time of the displacements produced by an earthquake. When combined with laboratory evidence of the failure of materials, these representations, or perhaps others not yet conceived, may lead to a coherent theory of the earthquake source.

The applicability of analysis of the properties of seismic body waves in either the time or frequency domains in elucidating the characteristic of the source has been demonstrated by a number of investigators. In these investigations, the displacement or strain field at large distances has been the primary object of inquiry. Recently, some opportunities to observe in detail the behavior close-in to the source have arisen, for example, in the Parkfield earthquake, and important new insights have been gained.