﻿Vol. 6 1.] TWIN-EARTHQUAKES. 23 



that the two parts of the Andalusian earthquake of 1884 were 

 respectively waves of direct and transverse vibrations, and they 

 account for the greater intensity of the second part by supposing 

 that the transverse vibrations were reinforced by reflected direct 

 vibrations. 



A distinct advance was made, 2 j^ears later, when Major 

 Dutton, relying on the distribution of damage, determined the 

 existence of two foci in the Charleston earthquake of 1886. In 

 the following year Prof. Mercalli, in his investigation of the Eivieran 

 earthquake of 1887, showed that the time-records and the obser- 

 vations on the direction of the shock implied the existence of two 

 submarine foci, one to the south of Oneglia and the other not far 

 from Nice. The same seismologist, in his valuable memoir on the 

 Calabro-Messinese earthquakes, concludes, from the form of the 

 isoseismals and the observations on the direction, that the earth- 

 quake of 1894 also originated in a double focus. I will now give 

 some reasons for believing that this view offers the best interpre- 

 tation of the evidence so far collected. 



In the first place, twin-earthquakes cannot be generally due to 

 reflection or refraction of the earth-waves at the bounding surfaces 

 of different rock-masses, although here and there the shock may 

 be duplicated in this manner. The wide area over which the 

 twin-shock is almost uniformly felt, shows that it is not a local 

 phenomenon . The existence of a sy nkinetic band, the almost-constant 

 brevity of the interval between the two parts, and the definite law 

 of variation in their relative intensity, duration, and period of 

 vibration, are equally opposed to a haphazard origin. Moreover, 

 on this theory, the first part of the shock should be the stronger, 

 for energy is lost by the reflection or refraction of a wave. And, 

 again, of* the earthquakes originating in a given district, a few are 

 twins and the majority simple ; yet, according to the explanation 

 here considered, the earth-waves should undergo deflection at the 

 same surfaces on every occasion. 1 



Nor can the two parts of a twin -earthquake be respectively 

 waves of direct and transverse vibrations ; for, if they were, there 

 would be no synkinetic band crossing the central district, and the 

 relative nature of the two parts would be uniform throughout the 

 disturbed area. Also, the interval between the two series does not 

 increase, as it should do, with the distance from the origin ; at 

 a distance of 170 miles it is only a few seconds, instead of 2 or 

 3 minutes, in length. The earthquake-sound accompanies both 

 series in precisely the same manner ; if the second series consisted 

 of transverse vibrations only, it would be unattended by any sound. 

 Both parts have been felt at sea, 2 and transverse vibrations cannot 

 be propagated in a liquid. Lastly, if the second series consisted of 

 transverse vibrations, every earthquake would be a twin-earthquake. 



1 A few, but by no means all, of the after-shocks of the Andalusian earth- 

 quake of 1884 were apparently twins. 



2 In the Ri?ieran earthquake of 1887, two strong shocks at a few seconds' 

 interval were felt on board a steamboat, on its way from Genoa to Marseilles. 



