﻿Vol. 6 1.] DERBY EARTHQUAKES OE 1904. 17 



V. Origin oe the Earthquakes. 



The elements of the originating fault, as determined by the 

 seismic evidence for both the principal earthquake and the after- 

 shock, are as follows : — (1) the mean direction of* the fault is from 

 N. 31° E. to S. 31° W. ; (2) its hade is to the north-west ; and (3) 

 the fault must pass through a point a short distance to the south- 

 east of the centre of the isoseismal 7, that is, it must pass through 

 or near the village of Hognaston. Comparing these elements with 

 those obtained for the fault in action in 1903, we see that they 

 are almost identical, the only difference being the slight one of 

 2 degrees in the mean direction of the fault. We may, therefore, 

 conclude with some probability that the two earthquakes 

 originated in slips along the same fault; and the pro- 

 bability seems strengthened when we consider that the two epi- 

 centres in both earthquakes were practically coincident. 



In 1903 there was no distinct preparation for the earthquake of 

 March 24th ; the impulses occurred simultaneously within the two 

 foci and were very nearly equal in strength, and there was no 

 sensible movement in the interfocal region of the fault. In less 

 than 4 hours two small slips took place in unknown parts of the 

 fault ; and 40 days later, on May 3rd, the increased stresses at 

 both ends of the interfocal region resulted in a slip chiefly within 

 that region, partly perhaps within the nearer lateral margins of 

 the foci, but closer to the surface than the two principal slips 

 of March 24th. 



In 1904, on the other hand, there was a preliminary slip within 

 the north-eastern or Wirksworth focus, followed in less than an hour 

 by the principal slips in both foci, that within the south-western 

 or Ashbourne focus being the more pronounced and succeeding the 

 other by about a second or even less. In this case the foci were 

 not completely detached, for throughout the interfocal region there 

 was a slight displacement, comparable in magnitude with that 

 which occurred in the marginal regions of the principal foci. 

 In other words, the nearer margins of these foci coalesced. About 

 8 hours later the increased stresses resulting from these move- 

 ments precipitated an interfocal slip, at about the same depth as 

 the previous slips in the principal foci. With this, the series of 

 movements seems to have terminated, unless there were two small 

 creeps a few hours later within the Wirksworth focus. 



EXPLANATION OF PLATE II. 



Map of the area affected by the principal Derby earthquake of July 3rd, 1904, 

 on the scale of 15 miles to the inch. 



[For the Discussion, see p. 33.] 



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



