﻿24 DR. CHARLES DAVISON ON [Feb. I905, 



Further, twin-earthquakes are not due to a repetition of the 

 impulse within the same, or an overlapping, focus : for, if so, the 

 order of relative intensity, etc., would not vary in a definite manner 

 throughout the disturbed area, the two parts of the shock would 

 never coalesce, and the mean duration of the interval would not 

 generally be confined between the limits of about 2 and 4 seconds. 



Thus, as twin-earthquakes are not due to the separation of the 

 waves arising from a single impulse, nor to repeated impulses in 

 the same focus or in overlapping foci, it follows that they must be 

 caused by impulses in two detached, or practically-detached, foci. 

 On this view of their origin, the phenomena described above admit 

 of a ready explanation : — 



(1) The twin-character of the shock would be perceptible as 

 far as the weaker of the two parts can be felt ; and the fact that 

 both parts are sometimes observed over the whole, or most, of the 

 disturbed area, shows that the two initial impulses were of nearly- 

 equal intensity. 



(2) In the neighbourhood of the epicentre corresponding to the 

 weaker impulse, the vibrations from that focus may be of greater 

 intensity than those from the more distant focus. The stronger 

 impulse does not necessarily occur first; and thus the order of 

 relative intensity may vary in different earthquakes, and in different 

 parts of the disturbed area of the same earthquake. I shall return 

 to this subject in the next section. 



(3) When, as seems to be generally the case, the interval between 

 the two parts is less than the time required to traverse the distance 

 between the two foci, the two series of vibrations must coalesce at 

 the surface, along a band passing between the two epicentres and 

 crossing the line that joins them approximately at right angles. 

 From the form of the synkinetic band we can determine which focus 

 was first in action. When the synkinetic band is straight, the two 

 impulses must have taken place simultaneously ; when the band is 

 curved, its convexity must face the focus first in action, for the 

 vibrations from that focus have to travel farther before they meet 

 the vibrations from the second focus. 



Y. Kelations between the Isoseismal Lines in a 

 Twin-Earthquake. 



In a twin-earthquake, observations of the intensity of the shock 

 refer, unless otherwise stated, to that of the stronger part ; and 

 thus, when the order of relative intensity varies throughout the 

 disturbed area, the isoseismal lines so determined are a combination 

 of the isoseismal lines corresponding to both foci. In such a case, the 

 isoseismals often present certain features which are characteristic of 

 their origin. 



Excentricity of the Resultant Isoseismal Lines. 



In a simple earthquake the isoseismal lines are roughly elliptical 

 in form, their longer axes are parallel, or nearly so, to the strike of 



