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



crest and trough were to become more pronounced, simultaneous 

 movements of the fold — the crest C to C and the trough T to T" — 

 would occur, and these movements would be accompanied by 

 a rotation of the median limb, the central portion M of which 

 would undergo no displacement. Thus, we should have two foci, 

 CC and TT", entirely separated by the interfocal region about M. 

 Moreover, the forces which give rise to an earthquake of this kind 

 must act at right angles to the direction of the fold, that is, parallel 

 to the direction of the fault. 



Fig. 3. 



It is evident that movements of this kind would suddenly increase 

 the stress within the median limb, and the effect of this increase 

 would be a sudden movement of translation of the median limb into 

 some such position as that indicated by the line C'M'T', thus giving 

 rise to a simple shock with its epicentre in the interfocal region of 

 the twin-earthquake. The interval between the twin-earthquake 

 and its first after-shock varies from a few hours to several weeks. 

 In the Stafford earthquake, the observations of the after-shock are 

 insufficient to determine its epicentre with precision ; but it was 

 probably due to a translation of the median limb of the fold about 

 fifteen hours after the occurrence of the twin-earthquake. 



On the Relations between the Stafford Earthquake oe 1916, 



the Derby Earthquakes op 1903, 1904, and 1906, and the 



Leicester Earthquakes of 1893 and 1904. 



On March 24, 1903, a strong twin-earthquake occurred in 



Derbyshire, 1 one focus being near Ashbourne and the other about 



3 miles west of Wirksworth. The distance between the foci 



is thus about 8 or 9 miles, and the direction of the longer 



axis of the isoseismal 7 is JN\ 33° E. This earthquake was followed 



on May 3 by an interfocal after-shock,' the direction of the longer 



axis of the isoseismal 5 being N. 25° E. On July 3, 1904, 2 another 



strong twin -earthquake occurred, in which the same foci were in 



action, the southern focus being 1^- miles east of Ashbourne, and 



the northern probably in the same position as in 1903. The 



1 Quart. Journ. Geol. Soc, vol. lx, pp. 215-32, 1904. 



2 Quart. Journ. Geol. Soc, vol. lxi, pp. 8-17, 1905. 



