14 THE FLOOR OF THE OCEAN 



FIGURE 5. DIAGRAMMATIC SEISMOGRAM, SHOWING ONSET OF PUSH (p), 

 SHAKE (s), AND LONG (l) WAVES. 



"finger" that traces on paper wavy lines from which the mo- 

 tions of the ground under the recording station can be deduced. 

 These motions indicate the successive passages of several kinds 

 of elastic waves. 



The first wave to arrive at a station is a Compressional 

 wave, analogous to that of sound in air. Here the motion of a 

 rock particle is to and fro, in the direction of wave propagation 

 or the wave-ray. Because the displacement of the myriads of 

 particles is in the to-and-fro sense, the first wave is called also 

 the Longitudinal wave. Being propagated by successive com- 

 pression and dilatation of the rock, it bears still a third name, 

 Push wave. In the diagram of Figure 5 the instant of arrival 

 of the Push wave is marked at the point P. 



Some time later a second wave makes its appearance; its 

 onset is marked at the point S. Now the motion of each rock 

 particle is across the wave-ray or direction of propagation. This 

 wave is accordingly named the Transverse or Shear wave. It is 

 analogous to the wave set up in a carpet when you shake it 

 from one end; hence a third name, Shake wave. 



After another delay the seismographic "finger" writes the 

 beginning of yet another kind of motion, caused by the so- 

 called Main waves which occasion the widest swings of the 

 rock particles. The Main waves have periods of swing much 

 longer than the periods of P and S waves and are therefore 

 also called Long waves. Their beginning is marked on the 

 diagram at the letter L. As in the case of the other waves, the 



