Elastic Waves, with Seismological Applications. 8L 



proximate application. If such a phenomenon is met with 

 in the comparatively simple case of sound-waves in air, similar 

 phenomena are certain to exist in the more complex cases that 

 correspond to earth shakings. 



Another point which this explicit recognition of purely 

 elastic and quasi- elastic disturbances suggests is in relation to 

 the measurement of earthquake velocities by comparison of 

 the effects at distant stations. Thus the purely elastic tremors 

 felt at stations far distant from the centre of seismic disturb- 

 ance have probably not come as such directly therefrom. They 

 are, so to speak, the feebler descendants of the gwasZ-elastic 

 disturbances, which may have caused havoc at localities 

 nearer to the seismic centre. The initial elastic tremors felt 

 at these nearer stations will reach the further distant ones 

 with intensities so diminished as no longer to be appreciable. 

 Thus in the very usual method of timing the arrival of a 

 tremor by the blurring of an image reflected from the surface 

 of mercury, it is evident that the speed, as estimated between 

 two stations in the line of propagation of the disturbance, 

 must be somewhat smaller than the true value. For before 

 the particular tremors which sufficiently blurred the image at 

 the first station have reached the second one, their intensity 

 has become diminished. Hence the sufficient blurring of the 

 image at the second station is due to the diminished violence 

 of tremors which passed through the first station subsequently 

 to the blurring of the image there. Now the same reasoning 

 will apply with even greater force to other than mere tremors ; 

 and especially will it apply to the case of the propagation of 

 the quasi-elastic disturbances which constitute dangerous 

 earthquakes. 



If the views so far expressed are correct, there is no difficulty 

 in understanding the nature of earthquake-sounds. As already 

 pointed out, they are to be traced to rapid vertical vibrations 

 of the ground, so rapid as to be inappreciable on our seismo- 

 graphs. Sometimes they may be due to transverse vibrations 

 of walls caused by horizontal displacements of the ground ; 

 or, as suggested by Mallet, they may be transmitted through 

 the framework of the body. That these sounds should fre- 

 quently precede the coming of the true earthquake-shock is 

 simply due to the running ahead of the purely elastic waves. 

 The nature of the rock or soil through which these waves 

 proceed will have a powerful influence upon their final 

 intensity. Thus in soft rock viscosity will soon destroy the 

 vibrations of short period. In such circumstances there will 

 be less chance of hearing earthquake- sounds than when the 



Phil. Mag. 8 5. Vol. 48. No. 290. July 1899. G 



