232 THE DEEEY EARTHQUAKES OP 1903. [May 1 904. 



that the two slips of the twin-earthquake were not simultaneous, 

 the earlier and stronger impulse taking place in the north-western 

 focus. In the Derby earthquake, there was little, if any, preparatory 

 movement : the two impulses occurred simultaneously, and were 

 approximately equal in strength. The foci, the centres of which 

 were about 8 or 9 miles apart, were completely detached, so far as 

 any sensible movement in the intermediate region was concerned, 

 and they were probably small in their horizontal dimensions, the 

 amount of slip becoming rapidly evanescent towards both lateral 

 margins. On the same day, two other small slips took place, but 

 their localities are unknown. 



An important result of the double slip was a sudden increase of 

 stress in the regions of the fault-surface within and surrounding 

 the margins of both foci. The portion of the fault between the 

 foci, being affected by movements at each end, received the greatest 

 accession of effective stress, and consequently, on May 3rd, forty days 

 after the principal disturbance, a minor slip took place chiefly or 

 entirely within this region, partly perhaps intruding on the nearer 

 lateral margins of the two foci, and extending upwards to within 

 a short distance from the surface. 



It may be useful, in conclusion, to compare the succession of 

 movements along the Derbyshire fault with those which have been 

 the parents of other recent earthquake-series. The first Carlisle 

 earthquake of July 9th, 1901, was the result of slips in two principal 

 foci, the centres of which were about 23 miles apart, and of a 

 continuous, though less, displacement throughout the whole inter- 

 mediate region. About 20 minutes later, there followed a slip which 

 resembled that of May 3rd, 1903, in being complementary to the 

 principal displacement and affecting the fault-surface between the 

 two foci. 1 Again, the Inverness earthquake of September 18th, 

 1901, was succeeded by several after-shocks, the foci of the more 

 important of which gradually approached the surface. 2 A similar 

 decrease in depth characterized most of the numerous after-shocks 

 of the great Japanese earthquake of 1891 ; and, as we have seen, 

 the focus of the Derby earthquake of May 3rd, 1903, was much 

 closer to the surface than those of the principal shock. The 

 materials at our disposal are still too scanty to allow of general 

 conclusions being drawn. Future shocks may render manifest 

 other modes of displacement ; but I trust that I am not too 

 sanguine in thinking that the careful study of earthquakes such as 

 we experience in this country may, in time, reveal to us the laws 

 according to which faults grow. 



EXPLANATION OF PLATE XIX. 



Map of the Derby earthquake of March 24th, 1903, on the scale of about 

 15 miles to the inch. 



1 Quart. Journ. Geol. Soc. vol. lviii (1902) pp. 371-76. 



2 Ibid. pp. 377-79. 



