TIDES AND THE RIGIDITY OF THE EARTH n 



surface. Waves of this type are characterised by a horizontal 

 movement parallel to the direction of propagation, accompanied 

 by considerable vertical movement. The conclusion that the 

 maximum phase of seismic movement must be transmitted by 

 waves of this type seems inevitable. The phase of subsidence 

 might be supposed to be due to the frittering away of the 

 energy through internal friction; doubtless this cause plays a 

 part but it has been proved by Prof. H. Lamb that waves 

 which spread over a surface, as distinguished from waves which 

 travel through a body, are always prolonged in a kind of " tail," 

 showing a gradual diminution of intensity, quite independently 

 of any dissipation of the energy. The characteristic feature of 

 the initial phase of the large waves, viz. the transversality 

 of the horizontal displacement, can be explained only by taking 

 account of the heterogeneity of the Earth's substance. Waves 

 possessing this feature can travel through a superficial layer, 

 provided the rigidity of the subjacent material be greater than 

 that of the layer. 



By regarding the Earth as made up of a nucleus and a 

 moderately thick superficial layer or crust and attributing 

 suitable mechanical properties to the nucleus and to the crust, 

 we can arrive at a fairly consistent representation of the various 

 phenomena. The first and second phases of the preliminary 

 tremors are, in this representation, taken to be due respectively 

 to compressional and distortional waves which travel through 

 the body of the Earth and emerge at the surface. The initial 

 phase of the large waves is taken to indicate the passage of 

 waves of transverse horizontal displacement transmitted through 

 the crust ; the maximum phase to indicate the passage over 

 the surface of waves of Lord Rayleigh's type ; and the phase 

 of subsidence to be the expression of the tails in which both 

 these types of waves would necessarily be prolonged. The 

 values to be attributed to the physical quantities by which 

 the state of the parts is specified are not completely determinate, 

 a change in the assumed density, for instance, being accom- 

 panied by a change in the inferred rigidity. But the indeter- 

 minateness is confined within relatively narrow limits. The 

 order of magnitude of the rigidity required in the nucleus 

 or at least in its more central portion is about three times 

 the rigidity of steel. This value may seem very large; but, 

 when we reflect upon the enormous pressures which must be 



