File:The Quarterly journal of the Geological Society of London (1904) (14598354300).jpg

Title: The Quarterly journal of the Geological Society of London
Year: 1845 (1840s)
Authors: Geological Society of London
Subjects: Geology
Publisher: London [etc.]
Contributing Library: Smithsonian Institution Libraries
Digitizing Sponsor: Biodiversity Heritage Library

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at, in Lancashire, Ireland, andelsewhere, the motion was a gentle swaying several times to andfro. The average of 88 estimates of the duration of the shock is6| seconds. Seismographic Records. The Caernarvon earthquake was recorded by a Milne seismographat Bidston, near Birkenhead, and by an Omori horizontal pendulumat Birmingham. Bidston is 60 miles from the centre in the direction E. 24° N.Mr. W. E. Plummer, the director of the observatory, kindly informsme that the first movements took place at 10h 5,u 5* a.m. Theseparate oscillations of the pendulum are not shown on the diagram,but there seem, he says, to have been two distinct impulses, thesecond taking place at 10h 7m 30s. The amplitude of the dis-turbance was even less than in the case of the Derbv earthquake ofMarch 24th, 1903. Birmingham lies 111 miles E. 20J S. from the centre. The record,which is enlarged 9*75 times in fig. 1 (p. 236), gives the componentof the motion in the east-and-west direction; and, as the movement u2
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DCO o © Vol. 60.] THE CAERNARVON EARTHQUAKE OF 1903. 237 of the ground is magnified 13-7 times by the pendulum, it followsthat the enlarged diagram represents the actual motion multiplied by134. The whole movement is divisible, as usual, into three parts—the preliminary tremors, the principal vibrations, and the concludingundulations. The preliminary tremors are first perceptible at10b o^ 56* a.m. (Greenwich mean time), and lasted for 13 seconds.The enlarged diagram shows hardly any trace of them ; but whenthe original record is examined under the microscope, they appearas minute notches, 51 in number, on the trace. The average periodof the tremors was. therefore, a quarter of a second. The principalvibrations began at 10h 6 9V, and lasted 26 seconds. The totalnumber of vibrations is 40 ; but the first 19 are, as a rule, of muchgreater amplitude than the rest. They have an average period of•63 second, while that of the remaining 21 vibrations is 67 second.In the 2nd and 19th vibr

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