﻿18 DK. CHARLES DAVISON ON [Feb. I905, 



3. Twin-Eaethquakes. By Chaeles Davison, Sc.D., F.Gr.S. 

 (Read December 21st, 1904.) 



I. Inteoduction. 



The essential characteristic of a twin-earthquake is the existence 

 in the shock of two maxima of intensity connected by weaker 

 tremulous motion, or the division of the shock into two parts sepa- 

 rated by a brief interval of rest and quiet. This feature, however, 

 is not entirely peculiar to twin-earthquakes ; for, occasionally, one 

 earthquake is succeeded by another so rapidly as to simulate a 

 twin-earthquake in this respect. A closer investigation of the 

 phenomena shows,, as will be seen, that the two parts or maxima 

 of a twin-earthquake originate in two detached, or practically- 

 detached, foci ; whereas, in a double earthquake, the foci are either 

 coincident or overlapping. A further distinction, partly dependent 

 on the former, may be noted. In all parts of the disturbed area 

 the member of a double earthquake which occurs first is felt first. 

 In a twin-earthquake, on the other hand, the second impulse may, 

 but does not necessarily, occur before the vibrations from the first 

 focus have reached the other ; so that, over most of the disturbed 

 area, the vibrations first felt are those which come from the nearer 

 focus, whether that focus was first in action or not. In a double 

 earthquake the second shock is a consequence of the first ; in a 

 twin-earthquake each is independent of the other. In other words, 

 a double earthquake is the result of successive impulses ; a twin- 

 earthquake is due to a single generative effort. 



As examples of twin-earthquakes, may be mentioned the Col- 

 chester earthquake of April 22nd, 1884 ; the Cornwall earthquake 

 of May 17th, 1892 ; the Pembroke earthquakes of August 18th, 

 1892, and November 2nd, 1893 ; the Leicester earthquake of 

 August 4th, 1893 ; the Hereford earthquake of December 17th, 

 1896 ; the Carlisle earthquake of July 9th, 1901 ; and the Derby 

 earthquakes of March 24th, 1903, and July 3rd, 1904. Also, among 

 the shocks of other lands, the Neapolitan earthquake of 1857, the 

 Andalusian earthquake of 1884, the Charleston earthquake of 1886, 

 the Rivieran earthquake of 1887, and the Calabro-Messinese earth- 

 quake of 1894. The Cornish earthquakes of March 29th and 

 April 1st, 1898, are typical examples of double earthquakes. 1 It 

 is also possible that a few earthquakes may belong to both classes. 

 The Leicester earthquake of June 21st, 1904, was probably a twin, 

 so far as regards the shocks at 3.30 and 5.28 a.m., while the latter 

 shock was itself a double earthquake. 



During the last 16 years (1889-1904) the total number of 

 earthquakes recorded in Great Britain is 160. Of these, eight, 

 or 1 in 20, were twins. They include five out of the seven 



1 Quart. Journ. Geol. Soc. vol. lvi (1900) pp. 1-7. 



