The Quarterly journal of the Geological Society of London . oseismal 4, was notless than 98,000 square miles. The Pembroke earthquakes of1892 and 1893 disturbed areas of 44,860 and 63,600 squaremiles respectively, but neither was sufficiently strong to causeany appreciable injury to buildings. With the exception ofthe Hereford earthquake, the Swansea earthquake was thereforethe strongest felt in this country for more than twenty years. Nature of the Shock. To most observers, the shock appeared to consist of a singleseries of vibrations, gradually increasing in intensity and thendying away. Oth
The Quarterly journal of the Geological Society of London . oseismal 4, was notless than 98,000 square miles. The Pembroke earthquakes of1892 and 1893 disturbed areas of 44,860 and 63,600 squaremiles respectively, but neither was sufficiently strong to causeany appreciable injury to buildings. With the exception ofthe Hereford earthquake, the Swansea earthquake was thereforethe strongest felt in this country for more than twenty years. Nature of the Shock. To most observers, the shock appeared to consist of a singleseries of vibrations, gradually increasing in intensity and thendying away. Others, however, noticed two distinct parts, differingconsiderably in strength, and separated by a brief interval of restand 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*3seconds between successive pairs of isoseismals. The mean durationof the interval within the isoseismal 5 was 2-1 seconds, or the sameas that observed during the Derby earthquake of 1904. Q. J. G. S. No. 251. 2 b. Yol. 63.] THE SWANSEA. EAKTHQUAICE OF 1906. 355 In the Derby earthquake of 1903, the two parts of the shockwere nearly equal in strength, and both were perceived throughoutthe whole disturbed area and by two out of every three the Swansea earthquake, on the other hand, only 14 per cent,of the observers within the isoseismal 8, and 16 per cent, of thosewithin 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 itwas the weaker part of the shock, and not the discontinuity of thevibrations, that escaped observation, for the mean duration of theshock within the isoseismal 5 was 6*1 seconds according to thosewho observed two distinct parts, and 37 seconds according tothose who noticed only one. The relative intensity of the two parts of the shock is recordedby 76 observers, of whom 12 regarded the first part as the stronger,and 58 the second, wh
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