44 



INTERNATIONAL ASPECTS OF OCEANOGRAPHY 



and the very much less basic material in the upper 

 layer of the continents, are caused by reflection 

 and refraction of the energy which arrives there. 

 The vertical extent of these vertical surfaces cannot 

 be more than a few tens of kilometers, as the very 

 long waves seem to show no effect of the kind men- 

 tioned. 



Nevertheless these vertical discontinuities may 

 affect the conditions down to a few hundreds of 

 kilometers. Inv^estigations on the depths of foci 

 of earthquakes^ have shown that everywhere in the 

 earth depths of foci of not more than 40-50 km. 

 prevail. In many earthquake regions there are 

 found, in addition, foci at depths down to 100 km. 

 Still greater depths occur in some earthquake regions 

 as in the Hindu Kush (200-250 km.), in the south 

 Atlantic (about 150 km.), Central America (about 

 130 km.), eastern Mediterranean region (150-200 

 km.), and many regions surrounding the Pacific 

 Ocean. Earthquakes originating at depths of 

 three hundred km. and more, however, are found 

 only in a relatively narrow belt around the Pacific 

 Ocean. They have been located thus far in Man- 

 churia, Sea of Okhotsk, south of Japan (near 30°N., 

 140°E.), in the Central East Indies about Celebes, 

 in the Solomon Islands, the Fiji-Kermadec area, 

 and western South America, but not North America. 

 (See figure.) The greate.st depths, of between 600- 

 720 km. thus far have been found in almost all these 

 regions, but especially in the Fiji-Kermadec area 

 and in western South Anierica. In general the 

 distances from the Pacific Ocean increases with 

 increasing depth. In South America, for example, 

 the normal shocks are close to the coast, shocks 

 with depths between 100-250 km. are beneath the 

 Andes and a third group of shocks with depths 

 between 600-700 km. have been located east of the 

 Andes. It has been found, besides, that in general 

 the type of movement is the same regardless of 



depth. That means that if we have a movement 

 towards the north on one side of a fault near the 

 surface, the movement is also in general in a north- 

 erly direction on the same side at larger depths 

 The data available so far are rather scanty in some 

 areas; however, they leave no doubt about the fact 

 that the Pacific Ocean basin bears a unique relation 

 to the occurrence of deep shocks. No similar phe- 

 nomena have been observed around other ocean 

 basins nor at the bottom of the Pacific Ocean. 



If we summarize our results we find that the 

 region comprised within the limits of the Pacific 

 Ocean as given above has one kind of structure and 

 all other regions of the earth, perhaps excluding a 

 part of the Arctic basin, another. In these latter 

 parts of the earth (non-Pacific area) there is a 

 continental layer which consists of several shells. 

 Its thickness is about 40-50 km. under the conti- 

 nents but decreases towards the Atlantic and 

 probably the Indian Ocean, where its thickness is 

 of the order of 20 km. There is no indication that 

 the continents have broken during any geological 

 time and drifted apart ; however, our findings would 

 be in agreement with the assumption that in early 

 geological times the thickness of the continental 

 crust was different in many localities from what it is 

 today and that plastic flow in the continental crust 

 may have changed the distribution of land and sea 

 in the area including all continents and the Atlantic 

 and Indian Ocean. 



The basin of the Pacific Ocean proper is a unique 

 element of the earth's crust and its boundaries 

 affect the layers down to many hundreds of km. 

 As it is not evident how the continental crust could 

 have been removed in a gradual way from the Pacific 

 Ocean the conclusion seems to be probable that the 

 Pacific Ocean either never has had such a crust or 

 that it has been removed by a cosmic event. 



EARTHQUAKE EPICENTERS IN OCEANIC REGIONS AND ALONG 

 CONTINENTAL BORDERS 



Plates 24, 25, 26 



The accompanying charts are intended to give a 

 general idea about earthquake epicenters rather 

 than to present a map of specific shocks. The 

 following symbols are used: 



' B. Gutenberg and C. F. Richter, Depth and Geo- 

 graphical Distribution of Deep-focus Earthquakes. Paper, 

 presented at a joint session of the Geological Soc. of Amer- 

 ica, Cordill. Sect, and the Seismological Society of America 

 at Berkeley, April 10, 1937. 



O Strong shocks frequent in that region. In most cases 

 one symbol stands for a few or even many shocks! 



# Occasionally strong shocks in that region. 



O Occasionally medium size shocks (about like the 

 Long Beach shock), but strong shocks rare. 



-|- Occasional shocks, but no recent strong shocks. 



As in many cases the epicenters are not known 

 to a higher degree of accuracy each symbol refers 

 to a region with a radius of a few hundred km. An 



