346 



NATURE 



[August 7, 1890 



ON THE STUDY OF EARTHQUAKES IN 

 GREAT BRITAIN} 



THERE can be little doubt that the more important 

 contributions to our knowledge of earthquakes 

 must be made in countries like Switzerland, Italy, and 

 Japan ; countries where earthquakes are frequently oc- 

 curring, where, occasionally, they are so disastrous as to 

 arrest universal attention, and where, at the same time, 

 there are many skilled observers aided by a sympathetic 

 and intelligent public. In England, as every strong 

 shock shows, there is no lack of observers. But our 

 earthquakes that are strong enough to attract general 

 notice within the disturbed area are few and far between. 

 If we exclude special districts, like Comrie and the 

 Durham coast, we shall probably be well within the 

 mark in stating the average number recorded as less 

 than one a month. 



The number of earthquakes that occur in Great 

 Britain must, however, be far greater than this. From 

 various causes, many shocks that are felt are never 

 placed on record. Others, again, that might be felt, must 

 certainly pass unnoticed, for, wherever seismic studies are 

 newly organized, it is found that people become educated 

 in detecting earthquake-shocks. But, however skilful 

 observers may become, there must always be a large 

 number of shocks that never could be felt, either from 

 the small amplitude or the long period of their vibrations. 

 Even in Tokio, where they talk about earthquakes as we 

 in England talk about the weather, " the majority of 

 shocks pass unfelt by people, while seismographs register 

 them sufficiently to allow measurements " (S. Sekiya, 

 Japan Seism. Soc. Trans., vol. x. p. 59). 



There is every reason to conclude, then, that, with the 

 aid of simple time-recording seismoscopes, the earth- 

 quakes of Great Britain would be found sufficiently 

 numerous to repay a more careful and systematic study. 

 That we shall have to be content, as a rule, with observ- 

 ing shocks that would elsewhere be considered slight is 

 evident of course ; but in their very feebleness we possess 

 advantages which are not afforded by severer shocks of 

 other lands. Not only are the phenomena much less com- 

 plex ; but, not being unnerved by danger, the observer 

 is able to concentrate his attention on them more calmly 

 and completely. Still more important is the fact, and in 

 this lies their greatest value, that, the smaller the area 

 disturbed, the more nearly can the position of the epi- 

 centrum be determined. If, as is frequently the case, 

 the shock be felt only within a small circular area, we 

 cannot be far wrong in regarding the centre of that area 

 as the approximate site of the epicentrum. And thus we 

 easily obtain the solution of what, in a great earthquake, 

 is one of the most difficult and important of the problems 

 to be attacked. 



Methods of Study in Great Britain. — Owing to the 

 feebleness of our shocks, and their comparative rarity in 

 a given district, the methods of study employed by us 

 must clearly be different from, and inferior to, those 

 adopted with such signal success in Italy and Japan. 

 We can hardly expect, for instance, that costly recording 

 instruments will be widely used in this country ; for, 

 even in Italy, as Prof, de Rossi points out {Btill. del Vide. 

 ital., anno iv., 1877, p. 5), it has been found better to 

 have a large number of observatories containing cheap 

 and simple instruments, than a few equipped with 

 seismographs more perfect and refined. 



If, on the one hand, then, our methods of earthquake 

 study are limited by the nature of the shocks we experi- 

 ence, on the other .we possess advantages, apart from 

 those already mentioned, that are more or less wanting 



I A Paper by Charles Davison, M.A., King Edward's High School, 

 Birmingham; read before the Birmingham Philosophical Society on 

 February 5, 1890. A few passages added since the paper was read are 

 ■enclosed in brackets. 



NO. 1084, VOL. 42] 



in regions where the phenomenon attains a more destruct 

 tive and interesting development. For instance, mos- 

 parts of England at any rate are so densely populated 

 that we are able, almost wherever a shock occurs, to pro- 

 cure a large number of observations of very considerable 

 value. And again, in the ease and accuracy with which 

 we can regulate our clocks in the neighbourhood of every 

 large town, we have an aid in our work which is as 

 valuable as it is rare in foreign countries. These two 

 facts in particular I mention here because they form the 

 foundation of our two most promising methods of investi- 

 gation. 



Looking at earthquakes chiefly from a geological point 

 of view— that is, regarding them as mere incidents, but at 

 the same time delicate indices, of the progress of terres- 

 trial evolution — the prime object of our inquiries is in 

 every case to determine the position of the epicentrum, 

 and, if possible, that of the seismic focus. For this pur- 

 pose, we have three methods at our disposal, depending 

 severally upon observations of the direction, intensity, 

 and time of occurrence, of the shock in different parts of 

 the disturbed area. 



The first method is interesting historically from its 

 having been used by Mallet in the earliest scientificstudy 

 of an earthquake. But modern seismologists have with 

 good reason generally discarded it ; and, in any case, it 

 could hardly be employed with success in this country. 



The method of intensities is far more trustworthy, and 

 is attended with good results whenever the observations are 

 sufficiently numerous and made at places that are fairly 

 evenly distributed over the disturbed area. With the aid 

 of such a scale as that drawn up by MM. de Rossi and 

 Forel, the intensity of the shock at any point may be 

 roughly estimated. Then, drawing lines including all 

 places where the intensity is at or above a certain degree 

 of the scale, we obtain a series of lines of equal intensity 

 (isoseisnial lines), which, closing in towards the epicen- 

 trum, enable its position to be approximately determined. 

 For the slighter shocks that we experience, it would be 

 difficult to over-estimate the value of this method, the 

 only one that in certain cases can be applied. 



The last of the three methods, I think I may say, is 

 still upon its trial ; and if, so far, it has not yielded all 

 the results that are to be expected from it, I believe the 

 reason is that it has not yet been attempted in a country 

 where the conditions are so favourable for its application 

 as they are in many parts of England. What we require 

 for the purpose is, not a network of time-recording instru- 

 ments extending over the whole country, so much as a 

 moderate number suitably placed and regularly observed 

 in specially selected districts. If, by means of these 

 instruments or otherwise, the times of a shock can be 

 ascertained with accuracy at five or more places, these, 

 under certain conditions, are theoretically sufficient to 

 determine the position of the epicentrum, the depth of 

 the seismic forces, the velocity of the earth-wave, and, 

 consequently, the time of occurrence at the focus. And 

 it should be noticed that time-recorders in Great Britain 

 are practically free from the objection which attends them 

 in Japan and other regions where earthquakes frequently 

 last for one or several minutes. For, in such cases, 

 the character of the shock varies so greatly throughout 

 the disturbed area, that it need not, and probably will 

 not, be one and the same vibration which is registered 

 in different places, and considerable errors may thus be 

 introduced.^ If, then, we remember that our earthquake 

 shocks seldom last for more than a few seconds at most, 

 and that, in many parts of England and some parts of 

 Scotland, it should be possible to ascertain the time of 

 occurrence correctly to within a small fraction of a minute, 

 I think there can be little doubt that, for all but the 

 slightest shocks, a most fruitful method of earthquake 



' E. Knipping and H. M. Paul, Japan Seisra. Soc. Trans., vol. vi. p. 37 ; 

 also J. A. Ewing, vol. iii. pp. 63-54, and J. Milne, vol. iv. pp. loo-ioi. 



