﻿Vol. 6 I.] DERBY EARTHQUAKES OF 1904. 11 



two maxima, and this was no doubt due to the incomplete separa- 

 tion of the two parts, an intermediate tremor being observed within 

 the central area. Thus, at Ashbourne (1| miles from the centre of 

 the isoseismal 7), the shock consisted of a single series of vibrations, 

 which increased in intensity to a maximum, and then died away ; 

 at Sudbury (about 9 miles to the south), there were two such 

 maxima connected by weaker tremulous motion ; at Birmingham 

 (37 miles distant), two series of vibrations were felt, the first being 

 distinctly stronger, the second series a mere shudder. The inter- 

 mediate tremor was perceptible as far as Earnsfield near Southwell 

 (27 miles from the centre) ; while the twin-shock was felt to the 

 north at Bradford (54 miles), to the west at Ellesmere (50 miles), 

 to the south at Stourport (52 miles), and to the east at Hough 

 near Grantham (44 miles), or over an area of roughly 8000 square 

 miles in extent. 



This area is, however, traversed by a band within which the two 

 parts of the shock were superposed. It is difficult to trace the 

 boundaries of this band with accuracy, owing to the continuity of 

 the shock within the central district ; but the course of its median 

 line probably does not differ much from that indicated by the broken- 

 and-dotted line on the map (PI. II), especially towards the south- 

 east of the epicentre. This band differs in two respects from that 

 traced for the earthquake of 1903. In that case, the band was 

 rectilinear and about 5 miles in width ; in the earthquake of 1904, 

 it was hyperbolic in form, the concavity of- the curve facing the south 

 or south-west, and the width of the band is greater towards its 

 extremities, being 15 or 16 miles at a distance of 30 miles from the 

 centre. It will be noticed that the median line passes a short 

 distance to the north-east of the centre of the isoseismal 7. 



It is obvious from the distance to which the twin- shock was 

 perceptible that there was some, although not much, difference 

 between the intensities of the two parts. This conclusion is also 

 borne out by the observations on the relative intensity of the two 

 series or maxima. To the south-west of the hyperbolic band, 

 49 observers regarded the first part as the stronger, 9 the second, 

 while 2 thought them roughly equal in intensity ; to the north-east 

 of the hyperbolic band, the corresponding figures are 20, 10, and 1. 

 On the south-west side, however, 3 observers, and on the north-east 

 side 8 observers, were doubtful as to the order of intensity. "We may, 

 I think, conclude from these figures that, while the first part was 

 generally the stronger all over the disturbed area, the inequality 

 was less on the north-east, than on the south-west, side of the 

 hyperbolic band. 



Outside the hyperbolic band, the length of the interval between 

 the two parts varied from 1 to 4 seconds, The average of 75 

 estimates is 2*1 seconds, and the average is practically uniform 

 throughout the disturbed area, being 2'2 seconds within the iso- 

 seismal 6, 2*0 seconds between the isoseismals 6 and 5, and 2-1 

 seconds outside the latter isoseismal. 



