52 



INTERNATIONAL ASPECTS OF OCEANOGRAPHY 



Hemisphere. The interpretation of such data 

 doubtless will be important in geophysical and 

 geological research to advance understanding and 

 interpretation of Earth phenomena. 



For example, from observed earthquake-wave 

 velocities and reflections for different regions and 

 depths the crustal layer, which under most of the 



tribution of magnetic secular-variation agrees with , 

 that of land-areas (see figs. 4 and 5, after Fisk, 

 based upon data from 75° north to 65° south^) — 

 as witness the moderate rates of annual change 

 over the Pacific as compared with those over the 

 Atlantic and adjoining continental areas. Further 

 data bearing on correlation thus indicated between 



Fig. 4. Longitudinal Distribution of Proportion op Annual Change (AH/H) of Horizontal Intensity 



Fig. 5. L.\titudinal Distribution of Proportion of Annual Change {AH/H) op Horizontal Intensity 



continental and water-covered continental struc- 

 tures is about 25 km. thick, is indicated as either 

 lacking or quite thin under the Pacific Ocean includ- 

 ing possibly the Arctic region. Under the Atlantic 

 and Indian oceans this layer is of appreciable depth. 

 Thus under the Pacific Ocean the basic surface of 

 the Earth's mantle is practically exposed. There 

 then we may expect different geological and geo- 

 graphical properties from those found elsewhere. 

 Thus the observed longitudinal and latitudinal dis- 



the surface-distribution of the secular-change ac- 

 tivity promise conclusions concerning secular-varia- 

 tion processes localized in the crustal layer and 



^ In these graphs showing distribution of annual change 

 (AH/H) of magnetic horizontal intensity the lower curves 

 represent the average positive values of AH/H in each lune 

 between the meridians (upper figure) and between its paral- 

 lels at 20-degree intervals while the upper curves represent 

 the numerical magnitude of the average negative values. 

 Thus the shaded areas between the curves are measures 

 of the excess of the negative over the positive annual 

 change. 



