BRITISH EARTHQUAKES. 



Bv CHARLES DAVISON. Sc.D.. F.v .> 



E.\RTHyuAKi-:s, according to their nature and origin, 

 may be divided into three classes — simple, twin and 

 complex. In simple earthquakes the shock seldom 

 exceeds a few seconds in duration, and its intensitx' 

 increases to a maximum and then dies aw aw In 

 twin earthquakes the shock consists of two distinct 

 parts, separated by an interval of rest and quiet, 

 lasting, as a rule, for two or three seconds, each part 

 resembling a simple shock in nature and duration. 

 Complex earthquakes are usuallx- of considerable 

 duration and great violence. The\' ma\' last as long 

 as three or four minutes, and there are man\- 

 fluctuations of intensity and frequent changes of 

 direction. Corresponding to this difference in nature 

 there is also a diversitv 

 in origin. In simple 

 earthquakes the focus 

 consists of a single region, 

 near the centre of which 

 the initial impulse is 

 greater than elsewhere. 

 In twin earthquakes there 

 are two such regions, I-, 

 almost or completely de- FiGUKii 1. 

 tached from one another. 

 In complex earthquakes 

 many portions, which ma\ 



connected, and the violence of the shock is due 

 partly, as in simple and twin earthquakes, to the 

 friction of sliding rock-surfaces, partlv to the rapid 

 translation of the rock-masses themseK'es. In other 

 words, the movement which gives rise to the shock 

 is not as a rule permanentlv perceptible at the surface 

 in simple and twin earthquakes, while in complex 

 earthquakes it often remains manifest in the form of 

 fault-scarps and horizontal displacements. 



\Miate\-er mav ha\'e been the case in times jiast, 

 this countr\- is now. fortunateh', exempt from all 

 earthquakes of the complex order. Occasionally, 

 about once in ten years, a shock causes damage to 

 houses within a limited area, but the houses affected 

 are usually of an inferior class. The great majoritv 

 of our earthquakes are so slight that they would have 

 passed unnoticed if the\- had not occurred during the 

 hours devoted to rest and sleep. 



Frequency. 



During the last twenty-one years (1889-1909). in 

 which the greater part of my spare time has been 

 devoted to the study of British earthquakes, the total 

 number know n to me is 250, or almost exactl\- one a 



month. The number of disturbances described as 

 earthquakes in newspapers is of course considerabiy 

 larger, but many of these disturbances prove on 

 in\'estigation to be artificial or parth' artificial in 

 their origin. Many of them are caused h\ the firing 

 of heavy guns at a distance, their true character 

 being generally revealed by the explosive nature of 

 the sound, the apparent transmission of the waves 

 through the air and not through the ground, and by 

 the increasing confidence with which observers in 

 one direction attribute the shock and sound to gun- 

 firing. Others are due to the explosion of 

 meteorites ; a few. and their spurious origin is soon 

 detected, to the explosion of dynamite or powder 



magazines. An interest- 

 ing class of local shocks 

 is found to be confined to 

 mining districts. The\' 

 are often of considerable 

 intensity within a ver\' 

 small area, but are im- 

 perceptible at a distance 

 T.-. ot a tew miles from its 



A curve shewing the number of shocks felt centre. The\' ma\' some- 

 in this country at different hours. ji,^^^^ " ,^^ ^.^^^^^j " ^^. ^^^ 



fall of masses of rock from the roof of the 

 workings, but most of them ajjpear to be caused 

 by slips of the superincumbent strata along a fault- 

 surface, the slips being started by the withdrawal of 

 rock from the workings or b\- that of water in 



the focus consists of 

 or nia\' not be directh' 



pumping. During the last twenty-one }-ears not less 

 than seventeen local shocks are probabK- due to this 

 cause. 



Now, just as many slight shocks are wrongh' 

 confused with earthquakes and must be eliminated 

 as far as possible from our earthquake-catalogues, so 

 a large number may also escape detection, or, at any 

 rate, record. They may be attributed to artificial 

 operations, such as blasting, gun-firing, thunder, or 

 the passing of a distant train or vehicle. There is 

 other e\'idence than mere probable supposition. 

 The cur\'e in Figure 1 illustrates the number of 

 shocks felt in this country during the different hours 

 of the da}-. It show s that the\- are recorded more 

 frequentl}' during certain hours, and especialh' from 

 1 to 2 a.m., 4 to 5 a.m., 4 to 5 p.m., and 9 to 11 

 p.m. But the varying frequency is, in all probability, 

 more apparent than real, All earthquake-catalogues 

 founded on personal, and not instrumental, records, 

 show the same increase of frequency late in the 

 evening and in the early hours of the morning. It 

 is no doubt due to more favourable conditions of 



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