MODERN SEISMOLOGY SCRASE 199 



Love, who showed that another type of surface wave, in which the 

 displacement is horizontal and perpendicular to the direction of 

 propagation, can be transmitted in a layer of finite thickness resting 

 on a deep solid layer ; it is necessary that the velocity of distortional 

 waves should be greater in the lower layer. Love waves, as they 

 are called, are equivalent to horizontally polarized S waves reflected 

 up and down within the upper layer in much the same way as light 

 waves in a slab of glass, the upper face of which is a perfect reflector 

 and the lower face bounded by material of higher refractive index. 

 Since the displacement in these waves is wholly horizontal, they do 

 not appear on seismograms of the vertical component. The study 

 of Love waves has enabled Stoneley and others to obtain estimates 

 of the thickness of the continental upper layers; their results are in 

 reasonable agreement with those given by near earthquakes. Seismo- 

 logical evidence as to the structure of the crust under the oceans is 

 not so abundant, but the fact that surface waves travel appreciably 

 faster along oceanic paths than along continental paths indicates 

 that the granitic layer is thin or absent under the oceans; this fits 

 in with geological evidence which shows that granitic rocks are com- 

 paratively rare on oceanic islands. 



WAVES IN THE INTERIOR OF THE EARTH 



The waves of near earthquakes do not penetrate very deeply into 

 the earth and the information which they yield is confined to the 

 upper layers. The waves of distant earthquakes, on the other hand, 

 reach great depths, and they enable us to obtain some knowledge of 

 the constitution of the interior of the earth. In the first place, the 

 slopes of the time-distance curves of P and S indicate that the 

 velocities of these waves, which are about 8 and 4.5 kilometers per 

 second respectively just below the crust, increase with depth to about 

 13 and 7 kilometers per second at 2,900 kilometers below the surface. 

 Since the velocity depends directly on the elasticity and inversely as 

 the density of the material through which the wave is propagated, 

 it appears that the elasticity increases more rapidly with depth than 

 the density. The rate of change of velocity is not uniform all the 

 way down to 2,900 kilometers however ; the latest time-distance tables 

 of Jeffreys and Bullen indicate that there is a rapid increase of 

 velocity at a depth of about 400 kilometers for which there is at 

 present no satisfactory explanation. Further information about the 

 rate of change of velocity with depth has been obtained by study- 

 ing the variation of the amplitudes of recorded pulses with distance ; 

 by this method Gutenberg inferred that a discontinuity occurs at a 

 depth of about 1,200 kilometers. 



