﻿•Vol. 6 1.] TWIN-EARTHQUAKES. 33 



and synclines between 9 and 12 miles. The correspondence with 

 the average distance between twin-epicentres is thus close enough 

 to support or confirm the explanation given. 



A step in the growth of a crust-fold, such as that contemplated, 

 would leave the middle limb subjected at both ends to increased 

 stresses, which should, after a short interval, be relieved by a slip 

 occupying the whole of the interfocal region, and possibly intruding 

 on the areas of the twin-foci. 1 As these interfocal slips are simple 

 in character — the shock showing no signs of duplication — it is 

 probable that the movement of the middle limb is one of translation 

 and not, as in the principal displacement, of rotation : as if the 

 growth of the arches were followed by a much smaller bodily 

 advance of the crust-fold. 



On the view here given, it follows that simple and twin-earth- 

 quakes differ essentially in their origin. A simple earthquake is 

 caused by movements connected with a single system of folding. 

 The Caernarvon earthquake of 1903, for instance, was produced by 

 a fault-slip belonging to the Caledonian system, the Somerset earth- 

 quakes of 1893 and the Exmoor earthquake of 1894 by fault-slips 

 of the Charnian system. On the other hand, the parent-faults of 

 the Leicester earthquake of 1893 and the Hereford earthquake of 

 1896 belong to the Charnian system, while the earthquakes them- 

 selves were due to the growth of Caledonian folds ; and, in like 

 manner, the Colchester earthquake of 1884 and the Derby earth- 

 quakes of 1903 and 1904 were connected with the growth of 

 Charnian folds, the slips taking place along faults belonging to the 

 Caledonian system. Most of the earthquakes felt in this 

 country are thus merely incidents in the growth of 

 faults; and it is surely not without significance that 

 the strongest of all should be mainly due to the con- 

 tinued formation of some of our most important crust- 

 folds. 



Discussion on the foregoing three papers. 



Prof. Watts, referring to the paper on the Leicester earthquakes, 

 drew attention to the map of Charnwood Forest which he exhibited, 

 and described the general run of the faults observed by himself 

 The normal faults and thrusts on the north-east side of the anticline 

 probably hade to the north-east, while those on the opposite side 

 seem to hade south-westward. But the anticlinal fault which the 

 Author had selected as being the one that probably gave rise to 

 the two Leicester earthquakes might hade either way, and might 

 even hade north-eastward in part of its course and south-westward 

 elsewhere. He further referred to the conversation which he had 

 had with Prof. Lapworth on the third paper, who had pointed out 

 to him how twin-earthquakes occurring along the lines of Charnian 



1 In the Carlisle earthquake of 1901 the interfocal slip occurred after a lapse 

 of 22 minutes ; in the Derby earthquake of 1903 after 40 days, in that of 1904 

 after less than 8 hours ; and, probably, in the Pembroke earthquake of 1893 

 after 16 minutes. 



Q. J. G. S. No. 241. d 



