﻿Vol. 6 1.] TWIN-EARTHQUAKES. 31 



Colchester earthquake of 1884. If we may regard the two Leicester 

 earthquakes of 1904 as members of a twin-earthquake, the distance 

 would be about 12 miles ; and it is by no means impossible that 

 the Inverness earthquake of 1901 was a twin-earthquake, in which 

 the foci were so close as to give the impression of a double earth- 

 quake with overlapping foci. Excluding the two last earthquakes, 

 the average distance between the epicentres of recent twin-earth- 

 quakes in this country is 10 or 11 miles. 



Form of the Twin-Eoci. 



As a rule, the foci are elongated approximately in the direction of 

 the line joining them. In the Charleston earthquake, the epicentral 

 isoseismals in Mr. Earle Sloan's map are elongated in the neigh- 

 bourhood of both epicentres. Two of the strongest after-shocks of 

 the Bivieran earthquake wereconnected, according to Prof. Mercalli, 

 one with the Oneglia focus and the other with the Nice focus ; and 

 the longer axes of the disturbed areas of both are roughly parallel 

 to the line joining the foci. In the Leicester earthquake of 1893 

 and the Hereford earthquake of 1896, the axes of the excentric 

 isoseismals are nearly parallel to those of the isoseismals which 

 surround them. The Hereford earthquake of 1896, the Carlisle 

 earthquake of 1901, the Derby earthquakes of 1903 and 1904, 

 and probably also the Pembroke earthquake of 1893, were either 

 preceded or followed by shocks which originated in the interfocal 

 region of the fault and disturbed areas elongated in the same 

 directions as the inner isoseismals of the principal earthquakes. It 

 is difficult, therefore, to resist the conclusion that the two foci 

 are portions of one and the same fault. 



Relative Depth of the Twin-Eoci. 



Though estimates of the absolute depth of the seismic focus are 

 in every case subject to considerable error, there is some reason for 

 thinking that the two foci of a twin-earthquake may be situated at 

 different depths. Major Dutton estimates the depth of the Wood- 

 stock focus of the Charleston earthquake at about 12 miles, aud 

 that of the E-antowles focus at about 8 miles. Prof. Mercalli 

 believes that the Oneglia focus of the Rivieran earthquake lay at a 

 depth of about 10| miles ; while, from the rapid decay of the 

 vertical component of the motion, he infers that the Nice focus was 

 nearer to the surface. In both these cases the weaker impulse 

 seems to have been connected with the shallower focus. The 

 British earthquakes, in which the innermost isoseismal is excen- 

 tric, apparently lead to a different conclusion. The surrounding 

 isoseismals are not oval in form, and wider in the neighbourhood 

 of the excentric isoseismal than elsewhere, but nearly or quite 

 elliptical. This implies that the intensity of the weaker part of 

 the shock diminished more slowly outwards from the epicentre than 



