﻿Vol. 6 1.] TWIN-EAKTHQTJAKES. 21 



Mean Duration of the Interval between the 

 two Parts. 



In British earthquakes the duration of the interval between the 

 two parts rarely exceeds a few seconds in length. Taking, first, 

 the whole disturbed area, the mean duration of the interval was 

 2*1 seconds in the Derby earthquake of 1904 ; 2*3 seconds in the 

 Pembroke earthquake of 1893; 2'5 seconds in the Leicester earth- 

 quake of 1893 ; 3 seconds in the Pembroke earthquake of 1892, the 

 Carlisle earthquake of 1901, and the Derby earthquake of 1903 ; 

 and 3*6 seconds in the Hereford earthquake of 1896. Por the 

 earthquakes of other countries estimates are somewhat rare; but 

 the interval was only a few seconds in length in the Neapolitan, 

 Andalusian, and Bivieran earthquakes. In the Calabro-Messinese 

 earthquake of 1894 five estimates, varying from 1 to 3 seconds, 

 give an average of 2*1 seconds. The Charleston earthquake forms 

 an exception from this point of view ; for the two maxima were 

 separated by about 34 seconds at Charleston, and the mean duration 

 of the interval throughout the disturbed area was slightly less than 

 half a minute. 



Again, the mean duration of the interval varies but little at 

 different distances from the epicentre. In the central district of 

 the Hereford earthquake of 1896 it was 3*4 seconds, in the sur- 

 rounding zone 3*3 seconds, and in the outermost zone only 4*1 

 seconds. In the Derby earthquake of 1903, the mean duration 

 was 2*9 seconds within the isoseismal 6, 3*0 seconds between the 

 isoseismals 6 and 5, and 3*0 seconds beyond the latter isoseismal. 

 In the Derby earthquake of 1904 the averages for the corresponding 

 zones were 2'2, 2'0, and 2*1 seconds. In the Neapolitan earth- 

 quake, the duration was 1 or 2 seconds at Potenza, and a few 

 seconds at Naples ; in the Andalusian earthquake, 1 or 2 seconds 

 close to the epicentre and in the surrounding zone, and 3 or 4 

 seconds at Madrid, distant 170 miles. The slight increase with 

 the distance in some cases is no more than might be due to the 

 gradual extinction of the weak vibrations at the end of the first 

 series and beginning of the second; and it is therefore probable 

 that the mean interval between the maxima of intensity is practi- 

 cally constant at all distances from the centre. 



Coalescence of the two Parts. 



While, in most cases, the twin-shock is perceptible over the 

 greater part of the disturbed area, there may exist within it a 

 narrow band along which the two parts are no longer distinctly 

 separated, but coalesce and form a single continuous series of vibra- 

 tions. As the two movements are felt together in this band, 

 I propose to call it the synkinetic band, and the median line 

 of the band the synkinetic line. Such a band probably exists 

 in most, perhaps nearly all, British twin-earthquakes, but in only 

 three cases can it be definitely traced — in the Hereford earthquake 



