652 



NATURE 



[May 20, 1922 



at a very moderate depth below the surface, probably 

 seldom over ten miles, and usually less, that is to say, 

 within the limits of the solid outer crust of the earth ; 

 and in this region it is difficult to conceive of any 

 cause, sufficient to originate the elastic wave-motion 

 of the earthquake, other than the sudden fracture of 

 the solid rock, where strain has outgrown the power of 

 resistance. 



Apart from this general reasoning from observation, 

 there are cases on record where considerable displace- 

 ments, of the ground have been measured by the 

 comparison of careful and accurate surveys made before 

 and after the earthquake. In three of these — the 

 Cutch earthquake of 1819, the Sumatran of 1892, and 

 the Calif ornian of 1906 — the largest movements took 

 place close to the line of fracture, and in opposite 

 directions on opposite sides of it, the displacements 

 decreasing on either side till a region was reached in 

 which no change, from the condition before the earth- 

 quake, could be recognised. As this is precisely what 

 would take place if a solid body, capable of elastic 

 deformation, was strained until fracture took place, 

 the conclusion is justifiable that such was in fact the 

 origin of the dislocation and displacements. 



So far the conclusions, which may be drawn from 

 observation, as they have been briefly outlined, belong 

 rather to the domain of physics than of geology, but 

 when we go on to consider the cause to which the 

 strain is to be attributed, and more especially the rate 

 of growth, we are brought into contact with problems 

 and deductions which are intimately connected with 

 geology proper, and to which I propose to confine 

 attention in the remainder of this address. As regards 

 cause : this is usually attributed to what are known 

 as the tectonic processes, a term which may approxi- 

 mately be described as the processes by which the 

 folding and faulting of rocks were produced, and, in 

 accordance with this attribution, the class of earth- 

 quakes with which we are concerned is referred to as 

 tectonic. The rate of growth of strain has almost 

 invariably been accepted as very slow, yet when the 

 subject is looked into, it will be found that there is 

 really no evidence to support the acceptance ; in 

 part it must be attributed to the general belief that 

 all geological action is necessarily slow, and in part 

 to the conclusion that the Earth is a solid inert and 

 highly heated body, cooling slowly by radiation, with 

 the subsidiary deduction that all deformation of the 

 outer crust must be referred to contraction, consequent 

 on that slow cooling. The latter of these reasons is now 

 abandoned by those who forced it on us, and the former, 

 though true in general, must not be treated as an 

 unchangeable law, for there are many cases where 

 a process, slow on the average, and as a rule, is 

 occasionally subject to temporary acceleration of rate. 

 The evidence, too, which has been regarded as con- 

 firmatory of the slow growth of strain, is, in truth, 

 more properly described as an interpretation of observed 

 facts in accordance with an hypothesis. 



In the report on the Calif ornian earthquake of 1906, 

 for instance, the displacements caused by that earth- 

 quake and an earlier one in 1868 are explained by a 

 slow growth of strain, extending over a centur}^ or so, 

 partly relieved by fracture in 1868, and again in 1906. 



NO. 2742, VOL. 109] 



The argument is conclusive in so far as it shows that 

 the effects are consistent with the hypothesis, but it 

 was not noticed that they would be equally consistent 

 with a condition of quiescence throughout the whole 

 period, with the exception of two short intervals 

 immediately preceding the two shocks, respectively. 

 The same may be said of all the supposed evidence in 

 favour of a slow growth of strain ; it is true that in 

 those earthquakes which have been investigated in 

 detail, and in which the observations allow of any 

 definite conclusion, the indications point to the 

 conclusion that the proximate cause is fracture result- 

 ing from excessive strain, but there is in no case any 

 evidence of the rate at which that strain accumulated ; 

 nor is it possible that any such evidence could be 

 found. The after-effects may satisfactorily establish 

 the conclusion as to the cause, but they can give no 

 indication of the time occupied in preparation ; the 

 earthquake comes and passes, it leaves certain records 

 behind it, but these records would be the same whether 

 the preparatory growth of strain was secular or in- 

 stantaneous in duration. 



Yet the problem is not insoluble, for there is another 

 line of attack, which has only become practicable 

 within the last few years. If we regard the growth 

 of strain as continuous, there will be a certain increment 

 which will lead to fracture, earthquake, and partial 

 relief ; then with a further increment the process will 

 be repeated, and so we reach the concept of a mean- 

 strain interval for each shock, which may be regarded 

 as constant, on the average, for any given region, 

 provided that the average is taken over a sufficiently 

 long period. Any variation in the rate of growth 

 of strain must be accompanied by a corresponding 

 variation in the frequency of earthquakes. We have, 

 then, four quantities so related to each other that if 

 three of them are known the fourth can be deter- 

 mined. Of these four, two, namely the mean 

 frequency and the variation from that mean in any 

 chosen period, can be obtained from observation, and 

 if the variation from the mean rate of growth of strain 

 is also known, for the selected portion of the whole 

 period, that mean rate which is the object of search 

 can be obtained by a simple rule of three sum. 

 So that if there were any external cause which, 

 acting periodically and alternately in increase and 

 decrease of the rate of growth of strain, and if it were 

 possible to disentangle the variations due to this from 

 those due to other causes, we would have a means of 

 framing a numerical estimate of the general rate of 

 growth of strain. 



One such cause of periodic variation is to be found 

 in the tide-producing stresses set up by the sun and 

 the moon. It is true that many attempts have been 

 made at different times to detect some connection 

 between the frequency of earthquakes and the position 

 of the moon, and that no such connection has yet been 

 established, but these attempts have all been based 

 on very imperfect records. In time it may, perhaps, 

 be possible to apply to an earthquake record the method 

 of harmonic analysis, which has proved so fertile in 

 the case of the ocean tides, but the day is long distant 

 when a record of sufficient completeness will be avail- 

 able. Meanwhile there are some simpler relations, of 



