Vol. 60.] DERBY EARTHQUAKES OE 1003. 211* 



Throughout the whole disturbed area, the double shock is dis- 

 tinctly recorded by 68 per cent, of the observers. In some parts 

 this percentage rises to more than 80, especially in an elongated 

 district about 30 miles in length, lying along the continuation 

 towards the south-west of the major axis of the innermost isoseismal. 

 In no large area does it fall below 48. 



If, however, we plot the places where the double shock was felt, 

 and also those where only a single series of vibrations was observed,. 

 a definite law of variation is rendered evident. The single shock 

 was felt chiefly within a narrow rectilinear band, about 5 miles 

 wide, running centrally across the inner isoseismals in a direction 

 from \Y. 34° X. to E. 34 c S., that is, at right angles to the longer 

 axes of the isoseismals. 1 In the map (PI. XIX) the boundaries of this 

 band are represented by broken lines. Outside the band, the interval 

 between the two parts of the shock was one of rest and quiet, its 

 average length over the whole disturbed area being exactly 3 seconds. - 

 Close to the band (as at Derby) the interval was much shorter, 

 though still distinct ; while, within the band, the shock generally 

 appeared continuous, the ends of the two parts overlapping, 

 although near the epicentral area, and close to the boundaries of the 

 band elsewhere (as at Quarndon), two maxima of intensity were 

 frequently perceived. 



From the mere fact that the double shock was noticed at places 

 near the boundary of the disturbed area, it is evident that the two 

 parts were of nearly equal strength. If there had been any marked 

 difference, it would have been possible, with so large a number of 

 observations, to draw isoseismal lines for each part of the shock, 

 and thus to determine the positions of the two epicentres. As it 

 is, there is often considerable doubt as to which part was the 

 stronger. At Derby, for instance, the first part of the shock was 

 regarded as the stronger by 19 observers, and the second by 16 ; 

 while 5 considered the two parts as of approximately-equal in- 

 tensity. In the whole disturbed area, 61 per cent, of the observers 

 state that the first part was the stronger, and 39 per cent, the 

 second. Dividing the area into two portions by the axis of the 

 rectilinear band, 60 per cent, of the observers on the north-east 

 side, and 63 per cent, of those on the south-west side, regarded the 

 first part as the more intense. 



Origin of the Double Shock. 



It is evident, from these accounts, that the double shock owed 

 its origin to two distinct impulses of nearly-equal strength ; 



1 It should perhaps be mentioned that the boundaries of this band were 

 laid down without any reference to the previously-drawn isoseismal lines, and 

 before the approximate positions of the two epicentres were known. 



2 This is the average of 125 observations, estimates of 10 or more seconds 

 being omitted. 



3 This result, as will be seen from the following paragraphs, is due: (1) to 

 the approximate equality of the two impulses ; and (2) to their occurrence at the 

 same instant. In each half of the disturbed area, the vibrations which formed 

 the first part of the shock were those which came from the nearer focus. 



