On Twin Earthquakes. 389 



faults on the north-east side of the Charnwood anticlinal axis, 

 which appear to hade in the direction given. In the south-eastern 

 focus, the fault hades in the opposite direction, the change of hade 

 taking place a short distance to the south-east of Leicester. 



On June 21st, 1904, two shocks were felt : the first, a very 

 slight one, at about 3.30 a.m. : the second at 5.2S a.m. The epi- 

 centre of the earlier shock was in the neighbourhood of Markfield 

 and Groby, or near the south-eastern margin of the north-western 

 focus of 1893. The second shock disturbed an area of about 

 1200 square miles, and was a double shock, not a twin, for the foci 

 were overlapping. It originated in a fault running nearly north- 

 west and south-east, hading to the south-west, and passing a short 

 distance from Tugby, and therefore in all probability coincident 

 with the fault in action in 1893. The distance between the epi- 

 centres of the earthquakes of 1904 was about 12 miles. Thus, 

 the foci of 1904 appear to have occupied the nearer margins of the 

 foci of 1893. 



3. ' The Derby Earthquakes of July 3rd, 1904.' By Charles 

 Davison, Sc.D., E.G.S. 



Although weaker than the earthquake of March 24th, 1903, this 

 shock, owing to its occurrence at 3.21 on a Sunday afternoon, was 

 felt over a much wider area (about 25,000 square miles). As in 

 1903, the earthquake was a twin, the epicentres being almost 

 exactly coincident with those of that year, one being situated near 

 Ashbourne, and the other, about 6 or 7 miles from it, near Wirks- 

 worth and Matlock Bath. The impulse at the south-western or 

 Ashbourne focus was slightly stronger than the other, and took 

 place a second or two later. The principal slip was preceded by a 

 slighter one in the north-eastern focus at 2.28 p.m., and was followed, 

 as in 1903, though after a shorter interval, by a slip in the inter- 

 focal region of the fault, at 11.8 p.m. 



4. ' Twin-Earthquakes.' By Charles Davison, Sc.D., F.G.S. 



In a twin-earthquake, the shock consists of two maxima of 

 intensity, or of two distinct parts separated by a brief interval 

 of rest and quiet. In Great Britain, one in every twenty earth- 

 quakes is a twin, and our strongest shocks (the Colchester earth- 

 quake of 1884, the Hereford earthquake of 1896, etc.) belong to 

 the same class. 



The two parts of a twin-earthquake differ in their order of 

 intensity, both in different earthquakes and indifferent parts of the 

 disturbed area of the same earthquake. The interval between 

 the two parts varies on an average from 2 to 3| seconds ; and, 

 although the twin-shock is felt over a very wide area (sometimes . 

 over nearly the whole of the disturbed area), there may exist 

 within it a band (the synkinetic band) in which the two parts 

 coalesce aud form a single shock. 



These phenomena show that twin-earthquakes cannot be caused 

 by reflection or refraction of the earth-waves, nor by the separation 

 of the waves of direct and transverse vibrations, nor by the repetition 



