Dr. C. Davison — BritiHli Earthquakes. 303 



coincided with the northern focus of 1903 and 1904, lying about three 

 miles west of Wirksworth. 



Sound-phenomena. — Towards the east the sound-area extends as far 

 as the isoseismal 4, but in other directions the observations are not 

 sufficient to determine the course of its boundary. The sound was 

 heard by 88 per cent, of the observers within the isoseismal 5, and by 

 64 per cent, in the zone between the isoseismals 5 and 4 ; altogether 

 by 71 per cent, of the observers. It was compared to passing waggons, 

 etc., in 61 per cent, of the records, to thunder in 13 per cent., to wind 

 in 4, to loads of stones falling in 10, to the fall of a heavy body in 9, 

 and to explosions in 4 per cent. The beginning of the sound preceded 

 that of the shock in 54 per cent, of the records, coincided with it in 

 35, and followed it in 1 1 per cent. ; while the end of the sound pre- 

 ceded that of the shock in 24 per cent, of the records, coincided with 

 it in 48, and followed it in 28 per cent. The duration of the sound 

 was greater than that of the shock in 59 per cent, of the records, equal 

 to it in 37, and less than it in 4 per cent. 



Origin of the JEarthqiiaJce. — The three Derby earthquakes of 

 March 24th, 1903, July 3rd, 1904, and August 27th, 1906, in all 

 probability originated in the same twin foci, the epicentre of one being 

 about a mile east of Ashbourne and that of the other about three miles 

 west of Wirksworth, and the distance between them about seven or 

 eight miles. The earlier earthquakes were followed by slighter shocks, 

 the first after 40 days (on May 3rd, 1903) and the second after eight 

 hours, botli originating in the interlocal region of the fault. 



The longer axes of the inner isoseismals of the principal earthquakes 

 were directed JST. 33° E. in 1903 and jN". 31° E. in 1904; while those 

 of the inner isoseismals of the after-shocks were directed J^. 25° E. in 

 1903 and JN". 27° E. in 1904. In each of the principal earthquakes 

 the hade within the south-west or Ashbourne focus is to the north- 

 west. Now, if the hade within the north-east or Wirksworth focus 

 be towards the south-east, the greater expansion of the isoseismals for 

 each focus on the side towards which the fault hades would cause 

 a displacement of the north ends of the longer axes of the compound 

 isoseismals towards the east. Thus, the divergence between the 

 isoseismal axes of the princijjal earthquakes and the after-shocks shows 

 that the fault changes hade in the interlocal region, and that the true 

 mean direction of the originating fault is about JN". 26° E. This 

 agrees almost exactly with that deduced from the earthquake of 1906, 

 namely, JN. 25° E., the isoseismals of this earthquake being unaffected 

 by the impulse within the Wirksworth focus. 



In each of the three earthquakes the impulse within the south-west 

 focus was the stronger; but in 1903 the impulses occurred simul- 

 taneously, in 1904 the north-east focus, and in 1906 the south-west 

 focus, was first in action. The existence of a synkinetic band in each 

 case shows that all three were true twin earthquakes, the interval 

 between the impulses being less than the time occupied by the earth- 

 waves in traversing the interlocal region. 



7. Ohan Earthqualce : January llth, 1907. — Time of occurrence, 

 1.54 p.m. ; intensity not less than 6 ; centre of disturbed area in about 

 lat. 56° 26' K., long. 5° 21' W. ; number of records 57, from 36 places. 



