﻿Vol. 6 1.] 



TWIN-EARTHQUAKES. 



27 



strongly at the surface. The districts in which the first part of the 

 shock was the stronger are shaded by horizontal lines, and those 

 in which the second part was the stronger by vertical lines. 



If the interval between the impulses be greater than the time 

 taken by the earth-waves to travel from one focus to the other, and 



Fig. 2. — Variation of relative intensity in twin-earthquakes. 



the western focus be first in action, the distribution of relative 

 intensity will be that represented in fig. 3. If the eastern focus 

 be first in action, the order of relative intensity will simply be 

 reversed. 



On the other hand, if the interval between the impulses be less 

 than the time of transit from one focus to the other, a synkinetic 

 band will cross the disturbed area between the two epicentres. In 

 figs. 4-6 the synkinetic line is indicated by the hyperbolic curve. 

 In fig. 4 the western focus, and in fig. 5 the eastern focus, is 

 supposed to be first in action. Fig. 6 represents a modification of 



