260 



SCIEXCE-GOSSIP. 



THE CAUSE OF EARTHQUAKES. 

 By J. J. Stewart, B.A., B.Sc. 



'THE large number of earthquakes which have 

 *■ been occurring lately in various parts of the 

 world must have struck everyone. From thai 

 great district of earth-shakings, Japan and the 

 islands to the south towards Australia, to our 

 own earthquake region, the country round Comrie, 

 in Perthshire, a succession of shocks and earth 

 tremblings appears to be going on with but short 

 intervals of rest. This apparent increase of earth 

 movements is probably not so much a true increase 

 of frequency of earthquake shocks as an increase in 

 the records of those which occur. The world is cow 

 so much bound together by the electric telegraph and 

 the elaborate arrangements for the spread of news, 

 that a few hours after an earthquake has travelled 

 over a remote region we may read a more or less 

 full account of it in the newspapers at home. By 

 means of the sensitive instruments which have 

 been devised of recent years for the purpose of 

 indicating and recording earth tremors when the}' 

 occur, it is clearly shown that the surface of what 

 we call the firm and solid earth is rarely quiescent. 

 Tremblings and slight shakings which are too 

 slight to be noticed by the dwellers on the surface, 

 are yet noted and recorded by the seismographic 

 instruments, and such movements pass over the 

 earth in various places and in diverse directions 

 with remarkable frequency. Such instruments 

 also serve to show the far-reaching extent of those 

 surface disturbances. The instruments at Kew 

 Observatory indicate slight tremors of the ground 

 at the same period that great and striking earth- 

 quakes are traversing remote parts of Europe and 

 Asia. The waves of earth motion extend to our 

 country but with greatly diminished intensitv and 

 would not be noticed here at all, were it not for 

 the careful observations with these delicate pieces 

 of apparatus. 



It may be of interest to consider shortly what are 

 the causes which tend to produce these earth-shocks 

 and movements. As to the nature of earthquakes, 

 it seems certain that they consist of a wave motion 

 of the substance of the earth similar in kind to the 

 waves in the air by which sound is transmitted, or 

 to the waves of alternate extension and compression 

 by which a movement is propagated along an iron . 

 rod which is used to work a distant signal. The 

 shock or pull given in the signal-box to the end of 

 the rod travels along it with great rapiditv, and in 

 a time which seems to an observer so short as 

 to be instantaneous, the pull of the signalman 

 becomes a pull in the lever at the signal itself. 

 This rapidity of transmission is due to the fact 

 that waves of longitudinal extension travel very 



quickly in a metal like iron on account of its high 

 elasticity. 



The rapidity with which earthquake vibrations 

 travel through the ground has been measured, and 

 the velocity in different sorts of material such as 

 granite and sand has been investigated by experi- 

 ment. Air. Mallet found that the shock produced 

 by exploding gunpowder travelled at the rate of 951 

 feet per second in wet sand, and in solid granite at 

 the rate of 1,640 feet per second, or not far short of 

 twice as fast. From observations of the times at 

 which the same earthquake shock has passed 

 various places the speed of travelling in different 

 cases has been found to vary from about 600 feet 

 per second to about 3,000 feet per second. 



Amongst the earlier observations in this century 

 were those upon the form and nature of the 

 fractures and rents produced in buildings, and from 

 these the direction of the movements which take 

 place in earthquakes was estimated ; and not only 

 so, but from observing the direction of the gaps and 

 fissures in buildings over a considerable area after 

 it has been traversed by an earthquake the centre 

 of the disturbance can be approximately found, that 

 is the region below the surface whence the vibratory 

 motion has proceeded outwards in all directions. 

 Such observations are now replaced by the more 

 accurate ones obtainable by means of recently 

 devised seismographic apparatus. 



Earthquakes seem, certainly, to originate at some 

 definite spot or region of the earth's crust, generally 

 at no great depth below the surface. In various 

 cases of great earthquakes the centre of origin has 

 been found to be at a depth of from four to thirty 

 miles. What is the nature of the disturbance 

 which causes the sudden jar or shock giving rise to 

 an earthquake is still a question involved in much 

 obscurity. Various explanations and suggestions 

 have been offered, such as the sudden explosion of 

 steam from water under great compression. When 

 the water is suddenly relieved from the pressure 

 under which it existed it flashes into vapour with a 

 sudden expansion, rupturing the surrounding rocks 

 and causing a compression or blow which is 

 transmitted outwards to the rocks around. The 

 giving way of vast subterranean passages, due to 

 long-continued pressure of the surrounding rocks, 

 the explosion of gases, due to volcanic action and 

 such like causes have been suggested. Probably 

 each of these factors may be at work at different 

 times to produce earth tremors of greater or less 

 intensity ; but while the collapse of a cavity below 

 the earth's surface may, quite probably, be sufficient 

 to produce a local earthquake ; the larger and more 



