﻿20 DR. CHARLES DAVISON ON [Feb. I905, 



by 68 per cent, of the observers, and was perceptible over the 

 whole disturbed area of about 12,000 square miles. That of the 

 following year was felt over an area of about 8000 square miles, or 

 nearly one-third of the disturbed area. In the Andalusian earth- 

 quake of 1884 both parts of the shock were felt as far as Madrid, 

 which is 170 miles from the epicentre. In the Eivieran earthquake 

 of 1887 they were observed at many towns and villages in the 

 central zones, and even as far as Salo and Yicenza, which are 

 distant respectively 177 and 210 miles from the epicentre ; but the 

 weaker part was imperceptible in Switzerland and in other regions 

 near the boundary of the disturbed area. The twin-shock of the 

 Charleston earthquake of 1886 was felt at several places more than 

 600 miles away from the epicentre. 



Kelative Nature of the two Parts. 



As a general rule, the observations under this heading refer 

 to the relative intensity of the two parts of the shock. In the 

 Pembroke earthquake of 1892 the parts differed little in intensity ; 

 but, over all the land-area disturbed, the second seems to have been 

 slightly stronger than the first. In that of 1893, the second series 

 was the stronger near the western end of the isoseismal 6 and farther 

 west in Pembrokeshire, and the first in other parts of the area. 

 For the Hereford earthquake of 1896 the observations are more 

 detailed. These show that, in the north-western half of the dis- 

 turbed area, the first part of the shock was the stronger, of greater 

 duration, and consisted of vibrations of longer period ; that, in the 

 south-eastern half, the same features characterized the second part 

 of the shock ; and, moreover, that the boundary-line between these 

 two portions of the disturbed area was not straight, but concave 

 towards the south-east. In the Derby earthquake of 1903, the two 

 parts were so nearly equal in intensity that observers in the same 

 place differed greatly in their estimates. All over the disturbed 

 area, however, 61 per cent, of the observers state that the first 

 part was the stronger, and 39 per cent, the second ; and this pro- 

 portion was nearly the same on both sides of the minor axes of the 

 isoseismal lines. The first part of the Derby earthquake of 1904 

 was also generally regarded as the stronger, but the difference in 

 intensity between the two parts was clearly greater in the south- 

 western than in the north-eastern half of the disturbed area. In 

 the Charleston earthquake of 1886 the two parts were of nearly- 

 equal intensity, but at most places the first is described as the 

 stronger ; in the Eivieran earthquake of 1887, the second part of 

 the shock was the stronger all over the disturbed area, except 

 within a small area near Nice ; and in the Calabro-Messinese earth- 

 quake of 1894 the second part seems to have been everywhere 

 stronger than the first. Thus, not only does the order of relative 

 intensity vary in different earthquakes, but in the same earthquake 

 there may exist regions in which the order of intensity is reversed. 



