SHAPE AND STRUCTURE OF OCEAN BASINS 23 



ocean areas until surface wave methods were applied to the 

 problem. 



Surface Wave Studies 



Surface waves can gi\-e information about average crustal and 

 mantle conditions over the entire path traversed. Both Love and 

 Rayleigh waves have been used, and two independent solutions 

 have been obtained. Surface waves are easily observable over paths 

 of continental, oceanic, or even global dimensions. By choosing 

 purely continental and purely oceanic paths, it is possible to make 

 ready comparisons of crustal and mantle structure in the two 

 types of areas. 



The effective penetration of surface waves is the order of a wave 

 length. Their velocity of propagation depends upon a sort of 

 weighted average of the elastic properties of the materials down 

 to a depth of the order of a wavelength. By observing surface wave 

 velocity as a function of period, it should be possible to deduce 

 elastic properties as a function of depth. But the calculations are 

 difficult, for in a realistic treatment it is necessary to treat spherical 

 earth models with many layers, including gradient layers. These 

 calculations could not be made until new theoretical methods and 

 large electronic digital computers were available. Further, it was 

 necessary to make some improvements in seismographs to provide 

 suitable data on the longer period surface waves which could 

 "penetrate" several hundred kilometers into the mantle. Speci- 

 fically, long period vertical seismographs, used with matching 

 horizontals, were required in order to make definite segregation 

 and identification of the longer period Love and Rayleigh waves. 

 Finally, it was necessary to have some assurance that in its 

 topmost part the oceanic mantle had the same properties as the 

 continental mantle. This information, provided by seismic refrac- 

 tion measurements, was necessary as a starting point for the 

 surface wave calculations. Love waves of sufficiently long period, 

 called G waves or mantle Love waves, have been studied since 

 about 1926, principally by Gutenberg, Byerly, Imamura, and 

 Sato. Mantle Rayleigh waves were first described in 1954 (Ewing 

 and Press, 1954a, b), and their possibilities as a new and powerful 



