TJNDERTHRUSTING 345 



1. If the force be applied so that the block is pushed, then the latter 

 will behave much in the same manner as the upper block did when it was 

 considered the mobile element, except that the strength of the rock will 

 be greater and the limits of the secondary rupture will also be greater ; 

 that is to say, that the block below CD would be broken up with the de- 

 velopment of an imbricated structure. The imbrication of the block 

 below CD would, however, be restrained by the load of the block above it. 

 If this restraint were overcome, the block above CD would be correspond- 

 ingly broken, probably by normal faults; but as neither the imbrication 

 of the lower block nor the breaking of the upper block, by reason of a 

 stress applied as a push to the cross-section of the mobile block, would 

 produce the phenomena of large thrusting, the case will not further be 

 considered. 



2. If the force be applied to all parts of the mobile block below CD, 

 then the frictional resistance on CD may be incompetent to restrain the 



B 



-Diagrammatic Representation of a Thrust Block, showing Failure due to 

 underthrusting 



AB, surface of the earth ; CD. thrust plane. 



movement of the lower block, and the relative displacement may be in- 

 definite in amount, without imbrication or other breaking up of the lower 

 block. 



The stress transmitted, however, to the upper block by reason of fric- 

 tion on CD may generate a strain in that block which will exceed the 

 strength of the rock, and secondary ruptures will then be developed in it. 

 This stress will not be applied to the cross-section of the upper block, but 

 to its lower part, and will have a non-uniform distribution in the cross- 

 section. Under these conditions the strain in the upper block near CD 

 may exceed the strength of the rock in that region, while the strain at the 

 surface may be less than the strength of the rocks near the surface. Thus 

 the ruptures that relieve the strain may start at CD and be propagated 

 upward. If this happen, which seems probable, then these ruptures, by 

 reason of the progressive change in the direction of stress consequent on 

 progressive relief of strain, would be concave downward or toward the 



