306 Dr. C. Davison — The Stafford Earthquakes of 1916. 



It is clear, from the above account, that the two series of 

 vibrations were felt practically in all parts of the disturbed area, 

 but that in the curvilinear band, the two series were felt simul- 

 taneously and so appeared as a single shock either with one 

 maximum or two maxima close together. On this account, the 

 band is known as the synkinetic band. 



The existence of this band is evidence that the impulses in the 

 two foci occurred to closely together that the second focus was in 

 action before the vibrations from the first focus had time to reach it. 

 When the two impulses are simultaneous, the synkinetic band is 

 straight and crosses the axis of the central isoseismal at right 

 angles through its middle point. This was the case in the Derby 

 earthquake of 1903. If one impulse were to occur a second or so 

 before the other, then the waves from the focus first in action would 

 travel farther than the waves from the other focus before the two 

 series of vibrations coalesced. That is to say, the synkinetic band 

 would be slightly curved, it would pass nearer to the focus last in 

 action than to the other, and its convexity would be turned towards 

 the focus at which the first impulse occurred. From the form of 

 the synkinetic band, as shown by the broken lines in the map, we 

 conclude that the two impulses which formed the Stafford earth- 

 quake occurred almost simultaneously, but that the eastern focus 

 was in action very slightly — perhaps a second or two — before the 

 western focus. 



That the two impulses were very nearly equal in strength is clear 

 from the wide observation of the double shock. If they had differed 

 greatly in intensity, only the stronger part would have been felt 

 near the boundary of the disturbed area. This approximate equality 

 is also indicated by the observations on the relative intensity of the 

 two parts of the shock. For instance, in Staffordshire and the 

 adjoining counties of Shropshire and Derbyshire, on the east side of 

 the synkinetic band, 25 persons regarded the first part, and 20 persons 

 the second part, as the stronger; on the west side of the band, 

 21 persons regarded the first part, and 27 the second part, as the 

 stronger. Thus, it seems probable that, though the two impulses 

 were nearly equal, that of the eastern focus was slightly stronger 

 than that of the other. 



The synkinetic band has only been traced for four earthquakes. 

 In the Derby earthquake of 1903, the impulses were simultaneous 

 and practically equal in strength. In the Hereford earthquake of 

 1896, the Derby earthquake of 1904, and the Stafford earthquake of 

 1916, the foci were in action not quite simultaneously, and the 

 impulses were unequal in strength. 



The duration of the shock was estimated with care by 198 

 observers, and that of the interval between the two parts by 107 

 observers. The average of the former estimates is 3-1 seconds, and 

 of the latter 2-2 seconds. The figures in the following table (p. 307) 

 show how these averages vary with the distance from the origin. 



It should be mentioned that, in determining the average duration 

 of the shock, estimates when one part only was observed are 

 included. 



