August 1, 1896.] 



KNOWLEDGE. 



189 



HOW TO OBSERVE AN EARTHQUAKE. 



By Charles Davison, Sc.D., F.G.S. 



THE phenomena of earthquakes attain a very simple 

 form in this country. To realize this vre have 

 only to compare the severest of recent British 

 shocks with one of the great disturbances of other 

 lands: say the Essex earthquake of ls84 with the 

 Charleston earthquake of 1886 or the Japanese earthquake 

 of 1891. In the former case, though many buildings were 

 damaged and chimneys thrown down, the shock only 

 lasted a few seconds ; there were no great fissures in the 

 ground, no crumpling of railway lines, no changes in the 

 earth's surface features, such as the compression of river 

 valleys or changes of level along lines of fault. There 

 was some, though comparaiively little, derangement of 

 the underground water system. Two months after the 

 earthquake a single slight shock was felt, that would 

 hardly have attracted any notice had it not been for the 

 interest already aroused in the subject. How different is 

 this from the case in Japan ! During the day following 

 the disastrous earthquake of 1891 no less than three 

 hundred and eighteen shocks occurred. The daily number, 

 of course, rapidly declined ; but before little more than 

 two years had passed, as many as three thousand three 

 hundred and sixty-five shocks were recorded at an obser- 

 vatory situated a few miles from the chief centre of 

 disturbance. 



The simple character, the short duration, and the 

 isolation of British shocks are distinct aids to their obser- 

 vation ; the attention is not distracted by a multiplicity 

 of details. As the shocks are almost invariably slight, 

 panic and the consequent exaggeration of description are 

 to a great extent avoided. Their rarity is even, in one 

 sense, a point in their favour. This is especially the 

 case, as I have often found, when a seismologist endeavours 

 to collect information from observers in different places, 

 for those who have felt only one or two shocks in their 

 lifetime retain for long the vivid impression they produce. 

 On the other hand, previous inexperience of earthquakes 

 militates against their accurate observation. The shock 

 begins so suddenly, and is often of such brief duration, 

 that, almost before its true nature is recognized, it may 

 be over and all opportunity for detailed study gone. In 

 such cases, while some points stand out clearly enough and 

 can be easily described, especially with the aid of guiding 

 questions, others of perhaps equal importance escape notice, 

 or the recollection of them is afterwards too indistinct or 

 confused to be reproduced without uncertainty or error. 



It is not difficult, however, to attend to the principal 

 phenomena — those which will be of the greatest service in 

 determining the surface position of the centre of disturb- 

 ance and in throwing light on the nature and origin of the 

 earthquake. To describe these phenomena and to point 

 out others which are less deserving of notice may be of 

 some assistance to those who live in the districts which 

 are occasionally visited by earthquakes, and who are 

 desirous, when one does occur, of making the best use of 

 the brief time at their disposal. 



It should be mentioned at the outset that the nature or 

 order of the earthquake phenomena may vary much at 

 different points of the disturbed area. At a place not far 

 from the centre of the area, a low, rumblioLr sound is first 

 heard ; this Lrradually becomes louder, and after one or 

 more seconds a slight tremor is felt, both sound and move- 

 ment increase together in intensity, and then gradually die 

 away, the sound lasting a few seconds after the movement 

 ceases to be sensible. At its commencement the tremor 

 resembles that produced in a building by a passing carriage 



or train, but, as it gets stronger, separate vibrations are 

 perceptible. There is, indeed, no real distinction, except 

 in ma.Ljnitude and duration, between the tremulous motion 

 and the perceptible vibrations. The movement sometimes, 

 but not always, ends with tremors like those at starting. 



At places near the boundary of the disturbed area the 

 phenomena are of a simpler nature. If the shock be a 

 strong one, and the disturbed area consequently large, it 

 is possible that no sound at all may be heard, and the 

 only thing observed is a more or less feeble movement. 

 But in most earthquakes in Great Britain the sound is heard 

 as far as the shock is felt. In such cases, at a place some dis- 

 tance from the centre, the sound may be heard first and may 

 cease entirely before any motion is felt, or, at any rate, 

 soon after it begins. The instant when the sound is loudest 

 thus precedes the instant when the shock is strongest. 



Very frequently, after a pause of a few seconds, the 

 same phenomena are repeated with greater or less intensity, 

 the movement and sound nearly or quite dying away in 

 the interval. 



In describing the nature of the shock it may be con- 

 venient to distinguish the tremulous motion from the 

 principal vibrations, applying the latter term (principal 

 vibration) to each distinct and complete to-and-i'ro move- 

 ment. When the separate vibrations are so rapid and 

 small as to be imperceptible from one another, the term 

 "tremors" or "tremulous motion" should be used. 

 However carefully written a description may be, it will 

 always gain in value by the addition of a curve like that in 

 the accompanying figure. This curve shows, for the great 



Charleston earthquake of August 31st, 188G, how the 

 intensity varied during the course of the shock. 



Very often a sense of direction is perceptible, and in 

 many descriptions some importance seems to be attached 

 to this. If, however, these directions as observed at 

 different places are all plotted on a map (I am speaking 

 here of British earthquakes), they are found to be governed 

 by no definite law ; they do not diverge from any particular 

 point or area. One reason, no doubt, is that the direction 

 itself at any place may change during a shock ; but even 

 if this were not the case, the apparent direction within a 

 house is so largely governed by the direction in which the 

 house itself vibrates — i.e., by that of its longer axis — that 

 observations on direction are generally of little, if any, 

 value. It does not seem advisable, therefore, to pay much 

 attention to this point, and the time often given to it may, 

 as a rule, be more protitably bestowed. 



Another point on which valuable time is often wasted is 

 the determination of the time of occurrence. The first 

 impulse of an observer, when he begins to feel a shock, 

 seems to bo to pull out his watch. Now, if the watch were 

 very carefully regulated, or if its deviation from correct 

 (ireenwich time could be ascertained soon afterwards, it 

 would be of the highest importance to determine the time 

 of occurrence, especially if this could be done accurately to 

 within a few seconds. 15 at at the present day com- 

 paratively few watches can be depended on to this extent, 

 and not many persons are able within a few hours to com- 

 pare their watches with an accurately regulated clock. It is 

 seldom, indeed , that observations of the time areof any use for 

 determining the velocity with which the earth-wave travels. 

 It should be remembered, too. that an appearance of accuracy, 

 wliere none really exists, is easily given, and is most mis- 



