May 20, 1922] 



NATURE 



651 



turbances, only to be detected by very sensitive instru- 

 ments of special construction ; in some cases these are 

 clearly connected with great earthquakes — as the 

 word is here used— and by inference have been pre- 

 sumed to be so in all cases, even when there is no 

 independent evidence of the earthquake proper. The 

 records, regarded as records of the progressive enfeeble- 

 ment of the larger disturbance of the true earthquake, 

 would represent the cryptoseism, or unfelt earth- 

 quake, and be described correctly in the observatory 

 records as earthquakes. That they are correctly 

 so described is indisputable, if the word is taken in its 

 literal interpretation as a quaking, however feeble, 

 of the earth ; but if the implication is added that they 

 have the same origin as the greater disturbance, the 

 correctness of the description becomes doubtful. 



Some dozen years ago the results of a study of the 

 records of the Calif ornian earthquake of 1906 led me 

 to point out that, while the immediate origin of the 

 earthquake proper may be traced to occurrences which 

 take place in the outermost parts of the earth's crust, 

 these are but the secondary result of a deep-seated 

 origin, or bathyseism, which gives rise, at the same 

 time, to the disturbance which is recorded at long 

 distances by suitable instruments. Later work and 

 research has more and more confirmed both the correct- 

 ness of this interpretation and the conclusion that the 

 proximate cause, of great and destructive earthquakes, 

 is distinct from that of the long-distance records, 

 though the two origins are connected with each other 

 as effect and cause. 



In the present state of our ignorance of the nature 

 of the bathyseism, it is difficult to give a clear and 

 precise definition of the mode of connection between 

 it and the earthquake proper ; the subject is an inter- 

 esting one, and a review of the evidence, together with 

 the deductions which can be drawn from it, would fill 

 the time available, but it is not my intention to do 

 more than to attempt, by analogy, to illustrate and 

 explain the nature of the connection of the bathyseism 

 with its two independent results. 



Not many years have passed since, in the south- 

 eastern corner of England, we heard what were known 

 as the guns of Flanders ; and the description was 

 correct. The sound — it was more a sensation than a 

 sound — which was heard in Kent and Sussex was 

 undoubtedly produced by the report of great guns, by 

 the explosion, that is, of the charge in the gun itself ; 

 but had the explosion done no more than give rise 

 to the sound waves which travelled far in every direction 

 it would have little troubled the enemy. Simultane- 

 ously, however, with the production of the report, and 

 by the same explosion, a projectile was sent flying 

 through the air which exploded after a trajectory of 

 some miles, causing the damage which was the purpose 

 of its despatch. The effect of this second explosion 

 was severe but local, and at a short distance away 

 neither sound nor shock was sensible. 



Here we have a very complete analogy ; the explosion 

 of the gun represents the bathyseism ; the report and 

 sound waves travelling afar, correspond to the dis- 

 turbance which, propagated through the substance 

 of the earth, gives rise to the long-distance records : 

 the explosion of the shell to those dislocations in the 

 outer crust which produce the destructive earthquake ; 



NO. 2742, VOL. 109] 



and the trajectory to the connection, of which the 

 character is as yet unknown, between the bathyseism 

 and the surface shock. 



If this interpretation be accepted, it becomes 

 evident that the distant records represent something 

 which is distinct from the earthquake, as originally 

 understood, and that the study of records, with the 

 deductions drawn from that study, have little or no 

 bearing on the problems of geology, as we usually limit 

 the scope of that science. It is otherwise with the 

 earthquake proper ; originating in, and affecting, the 

 outermost crust of the earth, it has long, and rightly, 

 been regarded as one of the departments of geology, 

 both as regards cause and character, and it is with this 

 aspect of the subject alone that I shall deal. 



The character of earthquakes is known to an extent 

 sufficient for my purpose ; they are elastic waves, 

 transmitted through the substance of the earth, not, 

 as was once supposed, merely waves of elastic com- 

 pression, but of most complicated character, and, in all 

 but a small minority of cases, nothing but this vibratory 

 movement, the orchesis, can be recognised. Occasion- 

 ally, however, and only in the case of some earthquakes 

 of destructive violence, there is also a bodily and 

 permanent displacement of the solid ground, and this 

 mass, or molar, movement has been distinguished as 

 the mochleusis of the earthquake, as distinct from the 

 elastic displacement, accompanied by return to the 

 original position, which constitutes the orchesis. Now 

 the elastic waves can only be initiated by some sudden 

 impulse or disturbance, such as might be produced by 

 the fracture of rock, and as, in those earthquakes where 

 mochleusis can be recognised, there is usually evidence 

 of sudden movement along some pre-existent fault- 

 plane, or of rending and Assuring of the solid rock, 

 faulting or fracturing has come to be regarded as the 

 cause from which the vibratory disturbance, propagated 

 through the unfractured rock, originates. 



This conclusion is supported by the fact that the 

 proximate origin of the shock can almost always be 

 placed at a moderate depth from the surface. It is, 

 unfortunately, impossible to give any precise figures, 

 for hone of the methods which have been suggested 

 for determining the depth of the origin can be trusted, 

 some because they depend on assumptions which the 

 progress of knowledge has shown to be erroneous, 

 others because they demand data which cannot be 

 supplied with the requisite precision, if at all ; but 

 there is another way in which some idea of the depth of 

 origin may be reached, based on the fact that there is 

 usually a well-defined area of maximum intensity of 

 shock, surrounded by regions of diminishing intensity, 

 as the distance from the central area increases. Since 

 the violence of the disturbance will decrease with the 

 increase of distance from the origin, it follows that, the 

 nearer the origin lies to the surface the more closely 

 does the variation of surface distance from t^e epicentre 

 approximate to the variation in actual distance from 

 the origin ; hence it is evident that the rate of variation 

 of intensity of the disturbance will give some notion of 

 the depth of the origin. In this way, quite apart from 

 any numerical estimates which have been made, it 

 becomes clear that, excluding a small minority of earth- 

 quakes which will be referred to later, the origin lies 



