Theoretical seismic wave propagation

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Abstract
During the last four years of U.S. research into theoretical seismology, effective techniques of computing synthetic seismograms have come into quite general use. Good agreement between a synthetic record and an instrumental record of ground motion (the raw data of seismology) implies a good understanding of the seismic source; of Earth structure; and of the theory of wave propagation relevant to the frequency content of the data. At periods T greater than about twenty seconds, there is now in general terms a fairly complete understanding of all the details in a seismogram, provided one ignores the (usually) minor effects of lateral variation of Earth structure. For 2 < T < 20 seconds, only limited portions of a seismogram (e.g., 15 seconds following a P‐arrival) may be fully understood, the limitations coming from noises in the data due to lateral scattering, and also from our ignorance of the source, of fine‐scale vertical Earth structure, and attenuation. At periods less than about two seconds, the understanding of seismograms is usually limited only to time windows within which identifiable pulses arrive. At these short periods, our knowledge of Earth structure is inadequate; scattering effects are stronger, but are poorly understood except for some progress in the context of seismic reflection prospecting within crustal layers.

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