EAKTHQUAKE MOTION DISCUSSED THEORETICALLY. 53 



the energy in a particle of tlie first shell at any particular 

 phase of the motion be Kj, and the energy in a particle of 

 the second shell Kg, these quantities are fco each other 

 inversely as the masses of the shells — that is, inversely as 

 the squares of the mean radii of the shells. 



In symbols, h ^ h. . . . . (1) 



Assuming that energy is dissipated, 



i; ^ v~-^^' ■ ' * ^^^ 



where y< 1 is the rate of dissipation of energy which is 

 assumed to be constant. 



Area of greatest Overturning Moment. — Although 

 the rate of dissipation of the impulsive effects of an 

 earthquake may follow a law like that just enumerated, 

 it must be remembered that if the depth of the origin is 

 comparable with the radius of the area which is shaken, 

 the maximum impulsive effect as exhibited by the actual 

 destruction on the surface may not be immediately 

 above the origin where buildings have simply been lifted 

 vertically up and down, but at some distance from this 

 point, where the impulsive effort has been more oblique. 



At the epicentrum we have the maximum of the true 

 intensity as measured by the acceleration of a particle, or 

 the height to which a body might be projected, but it 

 will be at some distance from this where we shall have 

 the maximum intensity as exhibited by an overturning 

 effort. 



This will be rendered clear by the following diagram. 



In the accompanying diagram let o be the origin of a 

 shock, and o C the seismic vertical equal to r. Let the 

 direct or normal shock emerge at c, Cj, Cg, and at the 

 angles 6^, S^, &c. 



