106 KwiNO [chap. 7 



Officer et al. (1959). One other such measurement is reported by Luskin and 

 Ewing {in lift.) for a profile midway between New York and Bermuda, and two 

 measurements are known for the area north of the Puerto Rico Trench (Drake, 

 in lift.; Hersey et al., 1952). Table I lists the data for these four measurements. 



Table I 



Compressional and Shear- Wave Velocities and Poisson's Ratio for 



Mantle Rocks 



Profile 



Water Vp, Vg, Poisson's 



depth, km km/see km/sec ratio 



Carib 26-56 5.00 8.16 4.62 0.27 



«V-16 (N.Y.-Bda.) 4.74 8.06 4.27 0.31 



„y_j fNorth of Puerto 5.36 8.08 4.24 0.31 



JRico Trench 5.36 8.06 4.27 0.31 



'^ Unreversed measiu-ement. 



It is not understood why transformed shear waves from the mantle are ob- 

 served so seldom. Although crustal shear waves are observed relatively much 

 more often, they too exhibit a marked variation in strength from one area to 

 another which is not obviously associated with geographical location, seismic 

 velocities or other observed factors (Ewing and Ewing, 1959). Whether the 

 propagation of shear waves is through the crust or through the mantle, the 

 transformation from compressional waves occurs at the base of the low-velocity 

 sediments, or possibly at some interface within the sediments. Therefore, the 

 strength of any shear-wave arrivals from the high-velocity layers must be 

 dependent on the character of this interface. It may be possible to account for 

 the relative strength of the mantle shear waves versus the crustal shear waves 

 by the dependence of transformed energy on the angle of incidence, which is 

 different for the two paths. 



Whatever the cause, few measurements of mantle shear waves have been 

 obtained from marine seismic -refraction studies. The average determination of 

 Poisson's ratio for the mantle from these measurements is higher than the 

 value 0.25 tyi^ically found for dense, high-velocity rocks. The discrepancy, 

 indicating a high value of the ratio Vp/ Vs, may not have any significance in view 

 of the scarcity of measurements, although two of the high values are from 

 particularly good profiles and it would be difficult to interpret them differently. 

 If future refraction studies give similar results, the low shear-wave velocity 

 would seem to indicate a significantly lower velocity immediately below the 

 crust-mantle interface than at a few kilometers deeper. This would not be in- 

 consistent with the compressional-wave data from refraction studies; because 

 the mantle p waves are always masked for a considerable distance beyond the 

 range associated with the critical angle, and an increase of velocity with depth 



