Reports and Proceedings — Geological Society of London. 85 



the fault in action in 1893. The distance between the epicentres 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, ScD., F.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 short interval, by a slip in the 

 interfocal 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 earthquakes 

 is a twin, and our strongest shocks (the Colchester earthquake 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 in different 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 and 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 

 of the impulse within the same or an overlapping focus. They 

 must therefore be due to impulses in two detached, or practically 

 detached, foci ; and it is shown that all the known phenomena of 

 twin earthquakes can be thus accounted for. 



In British twin earthquakes the distance between the epicentres 

 varies from 4 to 23 miles, the average for seven recent earthquakes 

 being between 10 and 11 miles. As a rule, the foci are elongated 

 approximately in the direction of the line joining them, showing 

 • that they are portions of the same fault. The foci appear to be 

 situated at different depths, and in two cases the fault probably 

 changes hade in the region between them. 



The existence of the synkinetic band and the brevity of the 

 interval between the two parts of the shock show that, when the 

 impulses are not absolutely simultaneous, the second takes place as 

 a, rule before the vibrations from the first focus have time to reach 



