8 DE. CHAKLES DAVISON ON THE [Feb. 1906, 



Humberstone (2 miles east of Leicester), the shock consisted of a 

 single series of vibrations, lasting from 3 to 4 seconds, the intensity 

 of which increased to a maximum and then died away. 



Owing perhaps to the variable percej)tion of the intervening 

 tremor, the estimates of the length of the interval between the two 

 parts of the shock lie between rather wide limits. The average of 

 110 estimates is 3^ seconds. There is, however, no evidence of the 

 existence of a synkinetic band, 1 within which the two parts of the 

 shock coalesced, bordered by bands in which the shock consisted of 

 two maxima of intensity. Thus, the interval between the occurrence 

 of the two impulses must have been greater than the time required 

 for the vibrations to travel from one focus to the other. 



That the two parts of the shock did not*differ greatly in intensity 

 is shown by the magnitude of the area over which the twin-shock 

 was felt, and also by the very variable testimony of observers with 

 regard to the relative intensity of the two parts, each part being 

 regarded as the stronger by approximately the same number of 

 observers. If we treat all the observations as of equal value, no law 

 in the distribution of relative intensity is apparent ; and it is only 

 by rejecting all records, except those made by observers who were 

 awake at the time and who evidently attended carefully to the 

 phenomena, that any such law can be ascertained. It is then seen 

 that the first part of the shock was slightly the stronger over the 

 larger part of the disturbed area ; while the second part was the 

 stronger within a small and nearly-circular area indicated by the 

 broken line (#) on the map of the earthquake (PI. II). This area, 

 which is about 20 miles in diameter, includes the centre of the 

 north-eastern portion of the isoseismal 7, its own centre lying about 

 6 miles to the north-east. 



Origin of-the Double Shock. 



It is evident that the two parts of the shock originated in two 

 detached foci ; though, in the intervening region of the fault, there 

 must have been a slight displacement sufficient to account for the 

 widely-felt tremor connecting the two series of vibrations. The 

 two epicentres cannot be far distant from the centres of the two 

 portions of the isoseismal 7, and are therefore separated by a dis- 

 tance of about 17 miles. It follows, also, from the relative intensity 

 of the two parts of the shock, that the impulse within the south- 

 western focus took place a few seconds before the other and that 

 it was slightly the stronger, the latter inference being confirmed by 

 the larger size of the south-western portion of the isoseismal 7; 

 further, that every point of the disturbed area was reached first 

 by the vibrations from the south-western focus. The second part of 

 the shock was the more intense within the circular region near the 

 north-eastern epicentre, owing to the proximity of the corresponding 

 focus. 2 



1 Quart. Journ. Geol. Soc. vol. Ixi (1905) p. 21. 



2 Ibid. p. 28, fig. 3. 



