42 



INTERNATIONAL ASPECTS OF OCEANOGRAPHY 



which are propagated along the surface of the 

 earth, also can be used to find the velocities of waves 

 in different regions. In a medium which is not 

 homogeneous, the velocity of surface waves depends 

 upon the period. Short waves are propagated only 

 in a thin layer, whereas the energy of long waves is 

 propagated in a thick layer. In general, a con- 

 siderable amount of the energy of these waves is 

 propagated in that part of the earth's crust with a 

 thickness several times as great as the wave-length. 

 With increasing depth the energy propagated by 

 elastic surface waves diminishes exponentially. If, 

 for example, we have two layers, the upper one with 

 a thickness of ten kilometers and a velocity of three 

 kilometers per second for transversal waves, the 

 lower, with a velocity of four kilometers per second, 

 surface shear waves with a period of one second 

 (wave-length of the order of three kilometers), 

 will be propagated with a velocity of three kilo- 

 meters per second; if the wave has a period of ten 

 seconds, the wave-length will be greater than the 

 thickness of the layer, so a noticeable part of the 

 energy will be propagated in the deeper layer, and 

 the velocity of the wave will be between three and 

 four kilometers per second. If, finally, we consider 

 a wave with a period of 60 seconds, the wave-length 

 (nearly 240 kilometers) will be large as compared 

 with the thickness of the layer, nearly all the energy 

 will be propagated in the deeper layer and the 

 velocity of this wave will be nearly four kilometers 

 per second. As the whole matter is somewhat 

 complicated, we will not go into detail. 



If instead of two layers with constant velocity in 

 each we have a material in which the velocity in- 

 creases with depth, the effect will be similar; in this 

 case, too, the velocity of the waves will increase 

 with the period. In using this method B. Guten- 

 berg found in 1923 the difference in structure be- 

 tween the Pacific basin and all other regions of the 

 earth.' 



Combining the most recent data found from the 

 various investigations mentioned so far, Gutenberg 

 and Richter* arrived at the following conclusions: 

 The crust of the earth is divided in most regions 

 into several layers, the uppermost is the layer of 

 sedimentary rocks, with velocities of longitudinal 

 waves from about 1 km./sec. in very unconsolidated 



' B. Gutenberg, Dispersion und Extinction von seismis- 

 chen Oberflachenwellen und der Aufbau der obersten Erd- 

 schichten. Physikal. Zeitschr. vol. 25 (1924) pp. 377-381. 



* B. GutenJDerg and C. F. Richter, On Seismic Waves 

 (Third Paper). Gerlands Beitrage zur Geophysik, vol. 47 

 (1936) pp. 73-131. 



recent material to at least 6 km./sec. in very old, 

 consolidated sediments. The thickness of the sedi- 

 mentary layer varies locally withui very wide limits; 

 it may be totally absent, or may extend to depths 

 of over 12 km. (Depths of this order have been 

 found in the Los Angeles Basin by the use of applied 

 seismic methods.) Beneath these sedimentary rocks 

 is a layer which in many cases is known to consist 

 of granitic rock, in which the velocity of longitudinal 

 waves is about 5.5 km./sec. In some regions the 

 sediments are directly underlain by basaltic rock; 

 where data are available, usually one or two deeper 

 layers have been recognized within the crust. 



The base of the granitic layer has been found, 

 in the continental regions where it has been studied, 

 at depths between 15 and 20 km. In these same 

 regions the total thickness of the crust (depth of the 

 first major discontinuity) has been found to be from 

 30 to 50 km. Relatively small values for this 

 thickness have been found for the southwestern 

 United States, western Europe, and northeastern 

 Japan; about average thicknesses occur in central 

 and western North America, and in South America. 

 The largest values found thus far are in the region 

 of the Alps. In the Atlantic and Indian Oceans, 

 the total thickness of the crust is only a fraction 

 of that on the continents; the seismological data 

 offer no evidence as to the nature of the rocks 

 composing the crust in these areas, but in both 

 oceans there still is a well-marked discontinuity 

 between the crustal rocks and the mantle. There 

 is no evident vertical discontinuity between these 

 oceans and the adjacent continents. 



In the region of the Pacific basui no marked dis- 

 continuity between crust and mantle exists; except 

 for local accumulations of erupted basaltic material, 

 it does not appear that the elastic constants near the 

 rock surface differ significantly from those in the 

 mantle. Data for the north polar basin definitely 

 indicate the existence of a considerable area with 

 properties similar to those of the Pacific basin. 



All available evidence indicates that a continental 

 type of structure exists in certain outlying areas of 

 the Pacific Ocean. This is the case in the Poly- 

 nesian region, including the area west of the Bonin, 

 Marianne, and Caroline Islands. Besides, there is 

 evidence for continental structure in a limited area 

 in the southeastern Pacific, at considerable distance 

 from the coast of South America. 



The problem, of what materials the various layers 

 consist has not been solved completely yet. In 

 crystalline rocks, velocities of 4^-6 km./sec. have 



