﻿12 DR. CHARLES DAVISON ON THE [Feb. 1905, 



Origin of the Double Shock. 



There can be no doubt, from these observations, that the two 

 parts of the shock originated in two foci lying along a line directed 

 approximately from N. 31° E. to S. 31° W. A slight movement, 

 however, took place in the intermediate region, sufficient to cause 

 the tremulous motion felt between the two principal parts of the 

 shock. If the impulses had taken place simultaneously, the two 

 series would, as in 1903, have been superposed along a rectilinear 

 band. As it is, the curvature of the hyperbolic band towards the 

 south-western focus shows that the vibrations from the north-eastern 

 focus travelled farther than those from the south-western focus 

 before the two series coalesced. In other words, although the two 

 impulses were nearly simultaneous, the north-eastern focus was in 

 action a short time, perhaps a second or less, before the other. 



In the present case, the positions of the two epicentres can be 

 determined more accurately than in the earthquake of 1903. The 

 south-western epicentre must coincide very nearly with the centre of 

 the isoseismal 7, that is, it must be about 1| miles east of Ashbourne. 

 The north-eastern epicentre probably lies near Wirksworth or 

 Matlock Bath, and about 6 or 7 miles from the other. 



Corroborative evidence of these determinations is afforded by 

 the observations on the apparent direction of the shock The sense 

 of direction is more appreciable in houses in which the principal 

 walls are at right angles to the direction of the epicentre ; and it 

 was found, in the case of the Hereford earthquake of 1896, that 

 the average of a large number of observed directions, rough though 

 they may be, points very nearly to the epicentre. For the recent 

 Derby earthquake, it is possible to determine such average directions 

 for two districts, one around Derby and the other around Nottingham, 

 these two towns lying close to, but on opposite sides of, the hyper- 

 bolic band. In the Derby district, the mean direction is from 

 W. 37° 1ST., or exactly in a line from the centre of the isoseismal 7 ; 

 in the Nottingham district, the mean direction is from W. 39° N., 

 and, produced backwards, this line passes through a point 3 miles 

 west of Wirksworth. 



Lastly, the existence of the isoseismal 7 around the south-western 

 epicentre and the practical absence of such a curve from the north- 

 eastern epicentre lead to the conclusion that the impulse at the 

 south-western focus was stronger, though not much stronger, than 

 the impulse at the north-eastern focus. Consequently, on the 

 south-west side of the hyperbolic band, the first part of the shock 

 was the stronger ; on the north-east side, the first part was as a 

 rule the stronger, owing to the proximity of the north-eastern focus 

 from which the first vibrations arrived. 



Seismographic Records. 



The seismographic records of the earthquake are of little interest, 

 and do not add to our knowledge of its nature. Mr. W. E. Plummer, 



