436 TERENCE T. QUIRKE 



In order to achieve displacement, 



P sin cos 6 

 a 



must be sufficiently greater than 



G sin tan v 



a 



to overcome the friction due to both the normal component of 

 thrust and the normal component of gravity. The friction will 

 become less per square unit of fault plane as slickensides and 

 other smooth surfaces are developed by movement. But the total 

 area of friction increases with displacement, and it follows from 

 the preceding formulas that friction is greatest in intensity where 

 0, the angle of displacement, is greatest. 



The fault plane decreases in steepness as displacement increases. — 

 Rupture being accomplished, as indicated in Figure 6B, the fault 

 plane varies from places of low dip far beneath the surface to 

 places of high dip nearer the surface. Where the angle is low the 

 tangential thrust is greatest, the normal component of thrust is 

 least, but the normal component of gravity is a maximum, and 

 the resistant, tangential component of gravity is a minimum. 

 Near the surface where the angles are high and the normal compo- 

 nent of gravity is low, the resistant tangential thrust due to gravity 

 is a maximum and the tangential component of the compres- 

 sive force is a minimum, but its frictional component is a maxi- 

 mum. In all these movements the compressive thrusts must be 

 dominant, otherwise the force of gravity and friction would pre- 

 vent displacement. Therefore, displacement being granted, the 

 effect of gravity is not vital in the discussion. Obviously, then, 

 the effects of thrust result in a maximum shear along low-angled 

 planes and a maximum friction at high-angled planes. 



In the general case of rupture represented in Figure 6B maxi- 

 mum friction is near the surface. The hanging wall of millions of 

 tons of rock moving several miles along this fault plane will result 

 in enormous abrasion, much as a glacier abrades greatly the stoss 

 sides of steep opposing hillsides, and this abrasion will be greatest 



