Vol. 62.'] DOXCASTER EARTHQUAKE OF 1905. 11 



From this table it follows: (1) that, as in previous British 

 earthquakes, there is no evidence of any difference in velocity of the 

 sound-waves and of the waves containing the larger vibrations which 

 form the sensible shock ; and (2) that the sound before and after the 

 shock was more prominent than usual, indicating that the marginal 

 regions of the foci were of comparatively -large dimensions in a 

 horizontal direction. 



IV. Origin of the Earthquake. 



The only known element of the fault in which the earthquake of 

 ] 902 originated is its average direction, which is E. 25° N. and 

 W. 25° S. Eor the earthquake of 1905, little more can be deter- 

 mined. The line joining the centres of the two portions of the 

 isoseismal 7 runs north-east and south-west, and the longer axis of 

 the isoseismal 6 is directed E. 38° N. and W. 38° S. It is probable 

 that the latter coincides the more nearly with the average direction 

 of the originating fault. If the isoseismal lines are accurately 

 drawn, we should infer that the fault hades towards the north-west 

 within the south-western focus and towards the south-east within 

 the north-eastern focus. The distances between the isoseismals 7 

 and 6 differ, however, by so small an amount that it seems to me more 

 reasonable to conclude that the originating fault is nearly vertical 

 in this portion of its course, though perhaps hading slightly to the 

 south-east in the neighbourhood of the north-eastern focus. It could 

 hardly be otherwise, if the earthquakes of 1902 and 1905 were 

 connected with the same parent-fault. 



Of the existence of this connection there is no direct proof; but 

 its probability may be inferred for several reasons: — (1) both 

 earthquakes occurred in a district seldom visited by such disturb- 

 ances : (2) the north-eastern epicentre of the earthquake of 1905 

 is very close to the minor axis of the earthquake of 1902; and 

 (3) the direction assigned to the originating fault in each case 

 is nearly the same. The displacement which gave rise to the 

 earthquake of 1902 would be more likely to lead up to a dis- 

 placement along the same fault than to one in another fanlt of the 

 same system. 



With regard to the fore-shock in 1905, there is but little evidence; 

 it may have originated in the interfocal region or in the north-western 

 focus, but the displacement causing it was too small to have had 

 much influence on the succeeding movements. The first of these 

 occurred in the south-western focus, and rapidly extended across the 

 interfocal region until another comparatively-large slip took place 

 within the north-eastern focus. 'Of the two principal movements, the 

 latter was slightly less pronounced than the former, possibly owing 

 to relief of stress three years before. There is no evidence of any 

 after-slip in the interfocal region ; but this may have been rendered 

 unnecessary by the movement that intervened between the two chief 

 impulses. 



A twin-earthquake, as I have endeavoured to show in a recent 



