THE STRUCTURE OF THE OCEAN BASINS AS 



INDICATED BY SEISMOLOGICAL DATA AND 



EARTHQUAKE EPICENTERS 



By B. GUTENBERG 



Batch Graduate School of Geological Sciences, California Institute of Technology, Pasadena, Calif., U.S.A. 



Contribution No. Z26 



THE STRUCTURE OF THE OCEAN BASINS AS INDICATED BY SEISMOLOGICAL DATA 



When an earthquake occurs, two different kinds 

 of waves are generated : waves which travel through 

 the interior of the earth (space waves), and waves 

 whose energy is propagated chiefly along surfaces 

 (surface waves). The records of both kinds of 

 waves can be used to study certain physical proper- 

 ties of the several layers of the earth, especially of 

 the earth's crust. 



According to theory and to observations there 

 are two different types of space waves : longitudinal 

 waves, caused by the propagation of changes in 

 volume (either compression or rarefaction, there 

 being no difference in propagation between these 

 cases), and shear waves (transversal waves), due to 

 the propagation of a shear. The velocities of the 

 longitudinal waves {V) and of the transversal waves 

 (v) are connected with the bulk modulus k, the 

 coefficient of rigidity ji, and the density d of the 

 material in which the wave is propagated, by 

 the following formulae: 



y2 



fc + lM 



V = 



From the seLsmograms we find the times of arrival 

 of the different phases. Further, in very many 

 cases we are able to calculate the position and depth 

 of the focus, and the time of origin. In such cases 

 we can find the travel time (time between occurrence 

 of the shock and the arrival of a certain phase at the 

 station), and plotting these travel times against the 

 distances, we get the "travel-time curves" which 

 allow us to find the velocities of the several kinds of 

 waves as a function of the depth. 



Unfortunately it is very difficult to get travel 

 times of near shocks whose waves run only through 



the material at the bottom of the ocean. To get 

 true velocities, the instrument must be in contact 

 with the material of the earth's crust beneath the 

 ocean. It is very difficult to state how far this is 

 true in the case of instruments installed on islands. 

 The only observations which may fulfill such condi- 

 tions to a certain degree, have been published by 

 Angenheister' using seismograms near shocks regis- 

 tered at Apia (Samoa). They show that both kinds 

 of forerunners arrive earlier than in other regions 

 considered so far, and they were the first indication 

 of the fact that there are large inequalities in the 

 earth's crust. 



Another way to find data on the differences in the 

 earth's crust has been suggested by B. Gutenberg 

 and C. F. Richter.^ The amplitudes of waves 

 reflected from the surface of the earth depend on 

 the velocities at the point of reflection, in addition 

 to other quantities. The observations show that 

 waves reflected at the bottom in the Pacific basin, 

 with the exception of a few limited areas, and in the 

 Polar basin show usually much smaller amplitudes 

 than waves reflected under otherwise equal condi- 

 tions in the continents, the Atlantic or Indian Ocean. 

 The maximum difference occurs for epicentral dis- 

 tances of about 5000 km. ; at distances of this order 

 Pacific reflections, on an average, have only about 

 J of the amplitudes of continental reflections, indi- 

 cating a higher velocity of waves in the surface 

 layers of the Pacific. 



The observations of surface waves, that is waves 



" Angenheister, G., Beobachtungen an pazifischen Beben. 

 Gottinger Naehrichten, 1921. 



2 B. Gutenberg and C. F. Richter, On Seismic Waves 

 (Second Paper). Gerlands Beitr. zur Geophysik, vol. 45 

 (1935) pp. 280-360. 



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