Recent evidence concerning the structure and composition of the Earth's mantle

Abstract

The detailed investigation of mantle structure from the dispersion of surface waves is a young but vigorous field of study. Observations have been accumulating rapidly in the past few years because of the wide-spread installation of long-period instruments. Modern methods of data analysis used in conjunction with high-speed digital computers have made it possible to determine dispersion with greater precision and over a broader spectrum than has previously been possible. Observations now extend out to the fundamental periods of free oscillations of the whole earth. Interpretation has lagged behind observation because of the difficulties inherent in the problem of dispersion over realistic models of a spherical earth. This problem is now well in hand and dispersion appropriate to the standard earth models suggested by earlier body waves studies has been calculated. Even with digital computers, however, the computations are so formidable that until recently only the most tentative efforts have been made to modify the standard earth structures to give a more satisfactory fit to the data. A review as recent as the one by Bolt in the preceding volume of this series was, of necessity, limited to a discussion of the various standard earth models with no attempt made to use the full power of surface waves as an independent technique. Recent developments have made detailed surface wave interpretations possible and new information, rather than generalized verification of old information, should be rapidly forthcoming. Project Mohole and the International Upper Mantle Project have focused the attention of many earth scientists on the upper mantle. Because of this renewed emphasis present information and speculation on the properties of the mantle based on a variety of sources is summarized and re-examined in some detail. This provides the guide-lines for potentially fruitful further research and points out the nature of some of the discrepancies and limitations in our present knowledge that may be resolved by the surface wave method.

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