Vol. 60.] "DERBY EARTHQUAKES OF 1903. 231 



Sound-Phenomena. 



The boundary of the sound-area is shown by the dotted curve in 

 fig. 2. Towards the south, its course is somewhat uncertain, but 

 it probably does not deviate by more than a fraction of a mile from 

 the position there laid down. The boundary, as drawn, is 24 miles 

 long, 17 miles wide, and contains about 320 square miles. The 

 sound was heard by 92 per cent, of all the observers. It was 

 compared to passing traction-engines, etc., in 45 per cent, of the 

 records, to thunder in 39 per cent., wind in 6, loads of stones falling 

 in 3, explosions in 3, and to miscellaneous sounds in 3 per cent. 

 The beginning of the sound is said to have preceded that of the 

 shock in 47 per cent, of the records, and to have coincided with it 

 in 53 per cent. ; while the end of the sound is said to have coincided 

 with that of the shock in 58 per cent, of the records, and followed 

 it in 42 per cent. Twelve observers noted the time-relations of 

 both terminal epochs ; according to six of them, the duration of 

 the sound was greater than, and according to the other six equal 

 to, that of the shock. Thus, in its nature, and in its time- 

 relations with the shock, the sound of this after-shock resembled 

 that wmich accompanies the typical slight earthquake. 



Origin of the Earthquakes. 



According to the seismic evidence, the mean direction of the 

 earthquake-fault must be N. 33° E. and S. 33° W., its hade must be 

 to the north-west, and the fault must either traverse the village of 

 Hognaston or pass a short distance to the south-east of it. On the 

 Geological Survey-map (sheet 72), no faults are marked in the 

 immediate neighbourhood of this place. The surface -rocks belong to 

 the Yoredale Series, except for an inlier of Carboniferous Limestone 

 between Kniveton and Bradbourne, which terminates towards the 

 west in two masses of toadstone. The faults that border these 

 masses, according to the Survey-map, were for the most part inserted 

 to account for the presence of the toadstone. About a mile west of 

 Hognaston, a few small faults, half a mile or less in length, occur ; 

 but none agrees, either in direction or position, with the fault 

 assigned by the seismic conditions. This fault, however, is roughly 

 parallel to the strike of the neighbouring rocks, and either dies out 

 before reaching the surface or, more probably, is obscured by the 

 superficial covering of Drift. 



From the phenomena described in the foregoing pages, the 

 succession of events during the recent disturbances ma}' be clearly 

 realized. Eor many years, possibly for more than a century, there 

 had been no movement of any consequence along the earthquake- 

 fault. During the previous twenty-four hours, there may have 

 been a few small creeps, but the evidence on this point is inde- 

 cisive ; and the principal slips took place at 1.30 p.m. on March 24th, 

 practically without any sensible preparation. It is perhaps worthy 

 of notice that the Hereford earthquake of 1896 was preceded by 

 several shocks, originating chiefly in the south-eastern focus ; and 



