A Study of the Seismograms of English Channel Earthquakes.

Abstract

A series of four rather severe earthquakes (intensity about 7 on the Rossi-Fore1 scale) affected the lands bordering the English Channel during the years 1925-27. The seismograms of the most severe of these, that of 1926 July 30, have been studied by Dr. H. Jeffreys.? During an examination by the writer of the personal accounts of these and other earthquakes in this area a few seismograms were examined and were a t first thought to be too slight to be worth detailed study. Nearly all the personal work was therefore carried out, perhaps fortunately, without reference to seismograms. It was later found that the few records to hand of the earthquake of 1927 February 17 fitted almost exactly the times found by Jeffreys for 1926 July 30. An attempt was therefore made to collect all available seismograms of earthquakes in the area with the exception of the one studied by Jeffreys. Abundant records were collected of those of I925 February I , 1927 February 17, and 1927 November 19, and a few of 1914 January 8, 1927 August 2 1 , and 1929 December 23. The records of the former three have been studied in detail, largely with reference to the results and theories of Jeffreys. The seismograms were first measured using a transparent millimetres scale and a powerful hand-lens. Since many of the records were exceedingly feeble, every sudden movement that a t all suggested the arrival of a new wave train was recorded. All pulses that were visible on two or more traces (separate components or different instruments) a t onc place or that were quite definite on one trace were retained. It was then found tha t the times of the phases for the earthquake of 1927 February 17 did in every case agree with Jeffreys's published times for the earlier shock within the limits of observational error, thus showing that the epicentres were practically identical. Moreover, nearly all the pulses found by Jeffreys were present, including some examples of all the six wave trains which he found, and in every case S,. The relative magnitude of the impulses also agreed roughly with his observations. S, was very much the largest except in the record of the vertical component a t Uccle where S was the largest, and the maximum amplitude was reached soon after its arrival and before the arrival of S, as determined from the horizontal records. This vertical record was the only one out of over sixty measured in the course of the present work in which the arrival of S, was completely swamped by earlier waves. I n the cases of 1925 February I and 1927 November 19 i t was necessary first t o find the epicentre. This was done by systematic