Yol. 63.] SWANSEA EABTHQUAKE OF 1906. 353 



implies the existence of a secondary focus lying to the east of the 

 principal focus near Swansea. 



The isoseismal 6 is 117 miles long from east to west, 79 miles 

 wide, contains 6700 square miles, and is separated from the 

 isoseismal 7 by 16 miles on the north side and 21 miles on the 

 south. The isoseismal 5 is 163 miles long from east to west, 130 

 miles wide, and 16,600 square miles in area. Its distance from the 

 isoseismal 6 is 32 miles on the north side and 19 miles on the south. 

 The isoseismal 4 is 237 miles long from east to west, 203 miles 

 wide, and contains 37,800 square miles. On the north side it is 

 42 miles from the isoseismal 5, and on the south side 31 miles. 

 The last isoseismal, of intensity 3, is 303 miles long from east to 

 west, 274 miles wide, and 66,700 square miles in area. Its 

 distance from the isoseismal 4 is 45 miles towards the north and 

 26 miles towards the south ; but, in the latter direction, the curve 

 may be inaccurately drawn, Penzance being the southernmost place 

 of observation. 



Outside the last-mentioned isoseismal are three places from which 

 I have received reports, namely, Termonfechin (near Drogheda, 

 Co. Louth), 20 miles north-west of the curve, and Harrogate and 

 Wakefield, which are 31 and 14 miles respectively to the north-east. 

 It is not impossible that the shock should have been perceived by 

 persons at rest in upper storeys ; but, owing to the great distance 

 and to the isolation of these places, the disturbed area of the 

 earthquake should, I think, be regarded as bounded by the isoseismal 

 3, and therefore as containing 66,700 square miles. 



During the last twenty years, there have been three earthquakes 

 of the same order of intensity as the Swansea earthquake. Of 

 these, the Hereford earthquake of 1896 occupies the first place, 

 both in the amount of damage to property and in the extent of its 

 disturbed area, which, as bounded by the isoseismal 4, was not 

 less than 98,000 square miles. The Pembroke earthquakes of 

 1892 and 1893 disturbed areas of 44,860 and 63,600 square 

 miles respectively, but neither was sufficiently strong to cause 

 any appreciable injury to buildings. With the exception of 

 the Hereford earthquake, the Swansea earthquake was therefore 

 the strongest felt in this country for more than twenty years. 



Nature of the Shock. 



To most observers, the shock appeared to consist of a single 

 series of vibrations, gradually increasing in intensity and then 

 dying away. Others, however, noticed two distinct parts, differing 

 considerably in strength, and separated by a brief interval of rest 

 and quiet. The mean duration of this interval is almost constant, 

 being 2-2 seconds within the isoseismal 8, and 2*1, 2-0, and 2*3 

 seconds between successive pairs of isoseismals. The mean duration 

 of the interval within the isoseismal 5 was 2-1 seconds, or the same 

 as that observed during the Derby earthquake of 1904. 



Q. J. G. S. No. 251. 2 b 



