Yol. 63.] THE SWANSEA. EAKTHQUAICE OF 1906. 355 



In the Derby earthquake of 1903, the two parts of the shock 

 were nearly equal in strength, and both were perceived throughout 

 the whole disturbed area and by two out of every three observers. 

 In the Swansea earthquake, on the other hand, only 14 per cent, 

 of the observers within the isoseismal 8, and 16 per cent, of those 

 within the isoseismal 5, that is, only one out of every six or seven, 

 detected the twin-character of the shock. It is clear also that it 

 was the weaker part of the shock, and not the discontinuity of the 

 vibrations, that escaped observation, for the mean duration of the 

 shock within the isoseismal 5 was 6*1 seconds according to those 

 who observed two distinct parts, and 3" 7 seconds according to 

 those who noticed only one. 



The relative intensity of the two parts of the shock is recorded 

 by 76 observers, of whom 12 regarded the first part as the stronger, 

 and 58 the second, while 6 considered them as approximately equal 

 in strength. The places at which the first part seemed the 

 stronger are grouped within an oval area, lying to the east of the 

 isoseismal 8, 23 miles long from east to west, and 22 miles wide. 

 Outside this area, the boundary of which is indicated by the broken 

 line in the map on p. 354, the second part was stronger than the first. 

 From this law of variation in the relative intensity, it follows that 

 a secondary focus must be situated not far from the western 

 boundary of the oval area, the greater intensity of the first part of 

 the shock within this area being due to its proximity to the 

 secondary focus ; and, moreover, that the corresponding impulse 

 was weaker than that within the Swansea focus, and took place a 

 few seconds earlier. The interval between the two impulses was, 

 however, so long that the vibrations from the eastern focus were 

 felt everywhere before those from the Swansea focus. 



The uniform precedence of the vibrations from the minor focus is 

 of service in determining the boundary of the area over which the 

 twin-shock was perceptible. If the stronger part of the shock had 

 been the earlier in one region, and the weaker part in another, 

 the weaker part would be more readily felt when it occurred after 

 the stronger, and thus the curve bounding the twin-shock area 

 would not be an isoseismal line of the weaker series of vibrations. 

 In the case of the Swansea earthquake, this difficulty is avoided, 

 a,ud the broken-and-dotted line on the map (PL XXY) may be 

 regarded as the curve at which the weaker part of the shock ceased 

 to be perceptible under ordinary conditions. This curve, it will be 

 seen, coincides roughly with the isoseismal 5 towards the north, 

 east, and south ; but, towards the west it falls short of the 

 isoseismal 5 by 27 miles. It is 135 miles long from east to west, 

 127 miles wide, and contains 13,500 square miles. Its centre is 

 situated about 5 miles north-east of Maesteg and 20 miles east of 

 the centre of the isoseismal 8. Owing to the size of the area 

 disturbed by the weaker part of the shock, this point may be distant 

 several miles from the epicentre of the weaker focus. 



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