76 Prof. C. Gr. Knott on Reflexion and Refraction of 



I should also be inclined to regard as absolutely futile any 

 attempt to infer the nature of the movement in which the 

 shock originates from the nature of the motion of any surface 

 particle. 



Even in the extremely simple case of an isotropic elastic 

 solid, we see how a single reflexion (and probably refraction) 

 is sufficient to alter the type of wave motion, or rather to 

 bring into existence the other type. How much more will 

 this be true in such a heterogeneous mass as we know the 

 earth's crust to be ! And if the large earth shiftings, which 

 certainly mean straining beyond the limits of elasticit} r , differ 

 essentially from the purely elastic disturbances we have just 

 been considering, it will not be in the direction of simplicity. 

 It seems reasonable to expect in these also somewhat analo- 

 gous, although much more complex, relations. Hence it 

 may safely be concluded that the existence of distortional or 

 transverse waves does not of necessity imply a faulting of 

 rocks, any more than that the existence of the other type 

 necessarily points to a rupture or an explosive increase of 

 pressure. In short, as observation has only too plainly 

 demonstrated, it seems vain to look for any certain separation 

 of the normal and transverse types of vibration. Only when 

 the origin of the disturbance is within a few miles of us, and 

 is at an insignificant depth below the earth's surface, can we 

 reasonably expect to find an appreciable separation of the 

 two types of waves*. 



At this stage we may very fitly consider the general 

 import of the assumption of the existence of these two types 

 of wave in earthquake motion. The assumption is tanta- 

 mount to regarding the earth's crust as isotropic. Such a 

 characteristic may safely be applied to surface soil ; so that, 

 in artificial earthquake experiments, such as Milne has 

 carried out, it may be an easy matter to distinguish the 

 normal vibrations as their wave outstrips that of the trans- 

 verse vibrations. But it is altogether out of the question to 

 regard any stratified rock as isotropic ; while as for non- 

 stratified rocks, their heterogeneity makes a theoretical dis- 

 cussion of their elastic properties impossible. By considera- 

 tion then of the elastic properties of homogeneous isotropic 

 media, we can only hope to get at best a glimpse into the 

 seismic darkness. And small though the present contribu- 

 tion may be to the vast problem of earthquake motion, it 

 surely will have some value if only it opens our eyes to the 



* [This statement requires modification in the light of recently 

 acquired knowledge regarding the transmission at great speeds of tremors 

 through thousands of miles of the earth's material.] 



