EARTHQUAKE MOTION DISCUSSED THEORETICALLY. 47 



moment we will consider to be a square column, to be 

 twisted, and thus have its sha'pe altered. When the 

 twisting force is taken off it seems evident that the 

 column would endeavour to untwist itself or regain its 

 original form. Now the force which a body offers against 

 a clmnge of t'o^ume maybe very different from that which 

 it offers against a change of form. 



In disturbances which take place in the rocky crust 

 of our earth, it would seem possible that we may have 

 vibrations set up which are either compressions and ex- 

 tensions or twistings and distortions. These may take 

 place separately or simuUaneously, or we may have resul- 

 tant motions due to their combination. 



The following are examples of possible causes which 

 might give rise to these different orders of disturbance: — 



1. Imagine a large area stretched by elevation until 

 it reaches the limit of its elasticity and cracks. After 

 cracking, in consequence of its elasticity, it will fly back 

 over the whole area like a broken spring, and each point 

 in the area will oscillate round its new position of equili- 

 brium. In this case there will be no waves of distortion 

 excepting near the end of the crack, where waves are 

 transmitted in a direction parallel to the fissure. 



2. The ground is broken and slips either up, down, 

 or sideways, as we see to have taken place in the pro- 

 duction of faults. Here we get distortion in the direction 

 of the movement, and waves are produced by the elastic 

 force of the rock, causing it to spring back from its dis- 

 torted form. In a case like this the production of a 

 fissure running north and south might give rise to north 

 and south vibrations, which would be propagated end on 

 towards the north and south, but broadside on towards 

 the east and west. With disturbances of this kind, on 

 account of the want of homogeneousness in the materials 



