SHAPE AND STRUCTURE OF OCEAN BASINS 29 



will be required in order to lower the calculated group velocity 

 maximum. [Compare calculations of Dorman et al. (1960) with 

 observations of Press and Ewing (1956)]. This alteration can be 

 produced by changes in shear velocity, in density, or in both. With 

 the same density distribution, the derived Rayleigh wave shear 

 velocities are consistently lower than those found for Love waves, 

 when no allowance is made for sphericity. 



Comparison of Rayleigh wave continental and oceanic solutions 

 shows a large velocity decrease for the oceans, between 60 and 

 120 km. Similarly, the Love waves require a sharp drop in shear 

 velocities under oceans between 50 and 100 km. 



This surface wave evidence is the most direct and representative 

 evidence obtained to date about differences between the normal 

 mantle structure of continents and oceans. It is now established 

 that (1) the mantle beneath the oceans is significantly different 

 from that beneath the continents and that (2) the low-velocity 

 channel is found at depths of about 50 km, as compared to about 

 100 km beneath continents. 



Other Studies 



A summary of present knowledge of the properties of the mantle, 

 based primarily on seismic studies of body waves and surface 

 waves, and concordant with measurements and calculations of 

 thermal, electrical, and other geophysical and geochemical 

 properties gives a broad picture of two main zones or shells which 

 are found beneath nearly the whole of the earth. These are clearly 

 indicated in the velocity-depth curves in Fig. 7. 



Beginning at the core-mantle boundary, the lower of these two 

 regions, from about 900-2700 km depth, coincident with BuUen's 

 region D (1953) has been shown by Birch (1952) to be remarkably 

 uniform in elastic properties and, inferentially, in composition. 

 Figure 11 shows Birch's reduced densities to be constant in this 

 region. 



It has also been inferred to be quite uniform in temperature by 

 all recent studies of mantle temperature as shown in Fig. 12. 

 McDonald (1957), Fig. 13, has also concluded that the electrical 

 conductivity is nearly uniform in this region. 



