NATURE 



165 



THURSDAY, AUGUST 11, 1910. 



RECENT EARTHQUAKE INVESTIGATIONS. 

 The California Earthquake of April 18, 1906. Report 

 of the State Earthquake Investigation Committee 

 (in two vols, and atlas). Vol. ii.. The Mechanics 

 of the Earthquake. By Harry F. Reid. Pp. viii + 

 192. (Washington : Carnegie Institution, tqio.) 



OX the avii"age a little earthquake occurs in the 

 world every fifteen minutes. Great earthquakes 

 occur on the average about every four days, but it is only 

 on rare occasions that they hit populated districts. 

 The majority of the latter originate beneath deep 

 oceans or in uninhabited mountain regions, and as 

 neither little fish nor wandering tribes write letters 

 to the Times, all we know about their occurrence 

 comes from the observations of enthusiastic seis- 

 mologists. Nineteen hundred and six, however, was 

 a bad year for humanity, and exhibitions of seismic 

 relief took place in many districts. On Januarjf 31 

 a great disaster occurred in Colombia. On April 4 

 many soldiers and natives lost their lives in the 

 Kangra Valley. On April 14 nearly 6000 houses 

 fell in Formosa, whilst four days later San 

 Francisco and other towns in Central California 

 were reduced to ruins. On June 14, Kingston, in 

 Jamaica, was badlv shattered, and in the autumn, 

 on .August 17, Valparaiso and Santiago fell. In con- 

 nection with the disaster at San Francisco, British 

 shareholders in insurance companies were called upon 

 for twelve million pounds, whilst towards the re- 

 construction in Kingston their contribution was two 

 millions. What they paid for the happenings in 

 other parts of the world I do not know, but it is 

 quite certain that attention was directed to the fact 

 that even the inhabitants of the British islands were not 

 entirely beyond the pale of the vagaries of Mater 

 Terra. 



In 1906 the proprietors of newspapers, law\-ers, ex- 

 pert witnesses, the vendors of building materials, con- 

 structors, and others may have regarded earthquakes 

 as blessings in disguise. The charitably disposed had 

 frequently opportunities to derive comfort from their 

 donations, while scientific bodies saw opportunity for 

 investigations. The International Seismological Asso- 

 ciation spent a very large sum in collecting and 

 reproducing seismograms relating to the earth move- 

 ments which had devastated Central Chile. The 

 Carnegie Institution of Washington not only carried 

 on similar work for the earthquake of California, but 

 by publishing three volumes based on the material 

 collected it has considerably extended our knowledge 

 connected with seismological observations. 



To the first two of these volumes, issued as part i. 

 and part ii., reference has been made already (Nature, 

 March 4, 1909, vol. Ixxx., p. 10). The second volume, 

 by Prof. H. F. Reid, of the Johns Hopkins Univer- 

 sity, which is now before us, treats of "The Mechanics 

 of the Earthquake." In the discussion on the origin 

 of the shock, this is shown to have taken place from 

 point to point along a line of fracture many miles in 

 length and with a variable depth. There are therefore 

 NO. 2128, VOL. 84] 



many times of origin, each of which depends upon the 

 particular point considered. This may be a seisnio- 

 metrical refinement, but Prof. Reid, by his insistence 

 on this, has done much towards the exact under- 

 standing of certain observations. In the discussion 

 on permanent displacements of the ground we are 

 shown that as the results of three surveys, the first 

 of which commenced in 185 1, there have been per- 

 manent displacements parallel to the length of a well- 

 known fault. The ground on the east side of this 

 fault has moved southwards, whilst that on the oppo- 

 site side of it has gone to the north. A part of this 

 displacement, no doubt, took place at the time of 

 the earthquake, but there are convincing reasons for 

 the belief that much of it took place gradually before 

 the earthquake. The ground, in fact, was bent before 

 it broke. By experiments with a stiff slab of jelly 

 across which a slight cut made by a knife represented 

 a line of fault, the nature of the strain which takes 

 place before and after an earthquake is illustrated. 

 The actual forces required to produce in solid rock the 

 observed distortions, which resulted in rupture, are 

 given in mechanical units. If the depth of the fault 

 was 125 miles, its length 270 miles, and the average 

 movement 13 feet, then the work done at the time of 

 rupture is estimated at 13 x 10'° foot-pounds. After 

 this energy was set free, seismographs throughout the 

 world were set in motion. 



A cause for the deforming forces which resulted in 

 these strains is sought for in the theory of isostacy, 

 which implies that the shifting of materials accom- 

 panying surface denudation is compensated for by a 

 sub-surface flow. By this flow a dragging force is 

 exerted upon the superincumbent crust, which from 

 time to time yields suddenly. To predict tectonic 

 earthquakes we should build a line of piers at right 

 angles to a fault line and determine from time to time 

 the difference in direction between these piers and 

 their relative levels. Such observations, whether they 

 did or did not prove of value as an assistance towards 

 earthquake prediction, it is extremely likely that they 

 would throw light upon certain branches of earth 

 physics. Rotary movements are considered at some 

 length, and the idea that they may be the result of 

 vibrations at "right angles is considered to be the one 

 offering the simplest explanation. 



A chapter of great interest, not only to the builder, 

 but to the mathematician, relates to the influence of a 

 foundation upon apparent intensity, this being most 

 pronounced upon alluvium. 



Part ii. of this volume is devoted to a critical de- 

 scription of seismograms obtained from stations in' 

 various parts of the world. This is followed by old 

 and new explanations for the apparent increase in the 

 duration of an earthquake as it travels. This is another 

 good chapter, but it might easily have been extended. 

 The discussion of the velocities with which different 

 wave types were propagated and the paths they may- 

 have followed has been worked out with great care> 

 and is distinctly instructive. The determination of 

 the distance of the origin of an earthquake, as is now 

 well known, depends upon the interval of time be- 

 tween the arrival of the first motion and the arri\^al of 



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