SHAPE AND STRUCTURE OF OCEAN" BASINS H 



velocity distribution for the continental crust-mantle obtained by 

 Gutenberg and by Lehmann from body wave data, both of which 

 include a region of low velocity in the upper mantle, are consistent 

 with the Rayleigh wave data . . . ". 



The detailed investigation of mantle structure from surface 

 wave dispersion data is a young but very active field of study. 

 Rapid progress has been made possible by the introduction of 

 high-speed computing machines. Many details of mantle structure 

 await further study. Outstanding among these is a discrepancy 

 between Love and Rayleigh wave results for shear velocities in the 

 low-velocity zone of the order of 0.1 km/sec. It seems certain that 

 small adjustments in the assumed values of density and Poisson's 

 ratio will lead to a velocity structure which satisfies the data of 

 body waves, Love waves, and Rayleigh waves. 



Dorman et al. (1960) also studied Rayleigh wave dispersion 

 across oceanic areas and concluded that "It is also clear that the 

 velocity distribution below the depth of the continental M-dis- 

 continuity cannot be the same under continents and oceans. 

 Instead, an oceanic model obtained by successive approximation 

 to oceanic Rayleigh wave dispersion data shows that the region 

 of low shear velocity extends to much shallower depths under the 

 oceans, plus being a much more prominent feature under oceans 

 than under continents." These results are in general agreement 

 with the results of Love wave calculations discussed in a previous 

 paragraph, and are shown in Fig. 9 as Dorman 8099 for oceans, 

 and Lehmann I and Gutenberg for continents. 



Figure 9 shows a summary of the velocity distributions which 

 have resulted from surface wave studies to date. It shows the 

 principal results and also serves for discussions of the unsolved 

 problems. 



The most important unsolved problem is the discrepancy 

 between the structure for continental mantle deduced from Love 

 waves and from Rayleigh waves. 



This discrepancy sets in only at a depth of about 100 km, the 

 upper limit of the regions of lower shear velocities. As is well known, 

 all seismic investigations, except those using Gutenberg's inflection 

 point method, whether by means of body waves. Love waves, or 



