108 



EWING 



[chap. 7 



average value undoubtedly represent experimental errors, but some probably 

 indicate inliomogeneity in the mantle material. Low velocities are consistently 

 found in certain regions near large topographic features such as islands, oceanic 

 ridges and continental margins. It is difficult to fuul a ])attern of occurrence of 

 the high velocities. 



25 



5 20 



=> 15 

 q: 



10 10 





O PACIFIC 

 D ATLANTIC 

 A CARIBBEAN StA 

 and 

 GULF OF MEXICO 



"k 



o 



u% 





"A"A 



SOa 



O 





_L 



J L 



_L 



-8 -7 -6 -5 -4 -3 -2 



ELEVATION (KM) 



Fig. 4. Crustal thicknesses in the Atlantic and Pacific Oceans, the Caribbean Sea and the 

 Gulf of Mexico as a function of sea-floor elevation. 



Measurement of the velocity structure in the immediate vicinity of the M 

 discontinuity is difficult because the critical angle arrivals are masked by 

 preceding crustal arrivals, but results from a few shear-wave observations give 

 some indication of a velocity gradient in the uppermost part of the mantle. 

 Possible evidence for gradient is given also by a graph of mantle velocity versus 

 depth of the M discontinuity. 



References 



Dorman, J., M. Ewing and J. Oliver, 1960. Study of shear-velocity distribution in the 



upper mantle by mantle Rayleigh waves. Bull. Seism. Soc. Amer., 50, 87-115. 

 Ewing, J. and M. Ewing, 1959. Seisnciic refraction measurements in the Atlantic Ocean 



basins, in the Mediterranean Sea, on the Mid-Atlantic Ridge, and in the Norwegian 



Sea. null. Geol. Soc. Amer., 70, 291-318. 

 Ewing, M. and B. C. Heezen, 1955. Puerto Rico Trench topographic and geophysical 



data. Geol. Soc. Amer. Spec. Paper 62, 255-268. 

 Ewing, M., G. H. Sutton and C. B. Officer, 1954. Seismic refraction measurements in the 



Atlantic Ocean. Part VI: Typical deep stations. Bull. Seism. Soc. Amer., 44, 21-.38. 

 Ewing, M. and J. L. Worzel, 1954. Gravity anomalies and structure of the West Indies, 



Part I. Bull. Geol. Soc. Amer., 65, 165. 

 Ewing, M., J. L. Worzel, D. B. Ericson and B. C. Heezen, 1955. Geophysical and geological 



investigations in the Gulf of Mexico, Part I. Geophysics, 20, 1-18. 

 Gaskell, T. F., M. N. Hill and J. C. Swallow, 1958. Seismic measurements made by H.M.S. 



Challenger in the Atlantic, Pacific and Indian oceans and in the Mediterranean Sea, 



1950-53. Phil. Trans. Roy. Soc. London, A251, 23-83. 



