24 HISTORY OF THE OCEANS 



method for in\'estigating upper mantle structure were pointed out. 



By 1956 it was becoming apparent that the G wave, in its 

 simplest undispersed form, was best observed for purely oceanic 

 paths, for which the fundamental Love mode has nearly constant 

 group velocity in the period range from 20 to 300 sec. The com- 

 parison of the G velocity, about 4.4 km/sec, with the velocity of 

 4,7 km/sec for mantle shear waves, obtained from short period 

 surface wave and seismic refraction studies, indicated, even 

 without detailed calculation, the necessity for a low-velocity 

 channel in the oceanic mantle (Press and Ewing, 1956). It was 

 then apparent that detailed calculation of surface wave dispersion 

 in realistic models would help greatly to solve the problems of 

 upper mantle structure. 



Sato (1958) used an integral equation to deduce the shear 

 velocity structure from the Love wave phase velocity curve. This 

 structure agreed remarkably well with the velocity distribution of 

 Jeffreys and Bullen( Bullard, 1954, Table 3, p. 83). The calculation 

 was made for a flat earth and, even so, was almost beyond the 

 scope of the desk calculator used. 



Landisman and Sato (1958) made calculations of dispersion for 

 Love waves in a flat earth model having two homogeneous layers, 

 overlying a half space with linear gradients in density and shear 

 modulus. They compared their calculations with observations for 

 Love waves from the magnitude 8 Sinkiang shock of 1931, recorded 

 in Japan and Canada, for continental paths, and concluded that 

 "the only shear velocity structures which are concordant with 

 observations of surface wave dispersion and body wave tra\'el 

 times are those containing a low velocity layer in the region from 

 roughly 100 to 200 km below the Earth's surface." This was the 

 first quantitative interpretation of Love wave dispersion which 

 required the existence of the low-velocity zone in the upper mantle. 

 The model used was realistic despite the small number of layers, 

 owing to the gradient in properties of the half space. The data 

 were for continental paths. The continental mantle structure they 

 deduced is shown in Fig. 9 as model 38 km XII. 



Press (1959) emphasized the fact that the velocity of G waves, 

 4.41 km/sec, is the same for oceanic and continental paths, and 



