﻿26 DR. CHARLES DAVISON ON [Feb. I905, 



Divergence of the Isoseismal Axes. 



The curves in fig. 1 (p. 25) illustrate another peculiarity of the 

 resultant isoseismal lines. If the fault hades in the same direction 

 in both foci, the axes of the resultant isoseismals will be approxi- 

 mately parallel. If, as assumed in the figure, the fault hades to the 

 north in the western focus and to the south in the eastern focus, 

 the isoseismals of the western focus will be farther apart on the 

 north side than on the south, while those of the eastern focus will 

 be farther apart on the south side than on the north. The axis of 

 the isoseismal 5 will therefore be directed eastward and westward, 

 and those of the resultant isoseismals 4 and 3 will run from a few 

 degrees north of west to a few degrees south of east. An example 

 of this divergence of the isoseismal axes will be seen in the 

 Leicester earthquake of 1893, in which the axis of the isoseismal 5 

 runs from W. 30° N". to E. 30° S., while those of the resultant 

 isoseismals 4 and 3 are, respectively, from W. 40° N. to E. 40° S. 

 and north-west to south-east. 



Areas of Opposite Relative Intensity. 



If the vibrations which proceed from the two foci differ initially 

 in their intensity, duration, and period, the disturbed area may be 

 divided into regions in which the order of relative intensity, etc., 

 varies. In the case of the period of vibration, and roughly also in 

 that of duration, the synkinetic line separates one such region from 

 the other. The variation of relative intensity is, however, more 

 complicated, as the intensity of either part depends on the distance 

 from the corresponding focus. 



In fig. 2 (p. 27) the points A & B indicate the two epicentres, A 

 corresponding to the focus in which the initial impulse was the 

 stronger. The isoseismal lines for each focus are represented, for 

 simplicity's sake, by circles. At the points where similar isoseismals 

 intersect, both parts of the shock are of the same intensity. The 

 broken line passing through these points thus separates the dis- 

 turbed area into two districts, within the smaller of which the 

 vibrations from the weaker focus are stronger than those from 

 the other. It is evident that the centre of this area does not 

 coincide with the epicentre B, but is displaced on the side away 

 from A; and that its boundary is practically an unclosed curve 

 when the initial impulses are of nearly-equal intensity. 



The order of relative intensity at any point of the disturbed area 

 depends on the order in which the impulses occur in the two foci, 

 and on the length of the interval that elapses between their occur- 

 rence. Pigs. 3-6 (p. 28) illustrate the different cases that may arise. 

 In each of these, the western focus is supposed to be that in which the 

 stronger impulse takes place, while the small circle bounds the area 

 within which the vibrations from the eastern focus were felt more 



