i 9 4 



NATURE 



[August io, 191 1 



PROBLEMS IN SEISMOLOGY. 



\ GENERAL report of the recent meeting of the Inter- 

 ■*""*■ national Association of Seismology was given in 

 Nature of July 27, including Prof. Schuster's presidential 

 address, which touched illuminatively upon the broad 

 problems of the physics of the earth's crust. The follow- 

 ing supplementary notes are intended to give a fuller 

 account of the many subjects which were discussed by the 

 seismologists. 



Particularly interesting was the report presented by Dr. 

 Hecker, formerly of Potsdam, now of Strassburg, on the 

 tidal strains produced in the crust of the earth. Hecker's 

 well-known results have been supplemented by similar 

 work carried out at Dorpat by A. Orloff, who also gave 

 an account of his measurements of the deformation of the 

 earth's crust under the action of the moon. The method 

 of both sets of experiments is to search for a lunar perio- 

 dicity in the movements of sensitive horizontal pendulums. 

 If the earth were rigid, these movements, in so far as 

 they are due to the influence of the moon on the vertical 

 at any locality, can be calculated. The experimental 

 results give, however, a deflection of the vertical which is 

 only/ two-thirds of the calculated value. This is explained 

 on the assumption that the earth's crust yields to the tidal 

 action of the moon. Hecker's and Orloff's results show, 

 moreover, that the force acting on the pendulum is a 

 larger fraction of the moon's force when it acts towards 

 the east or west than when it acts towards the north or 

 south. In other words, the earth seems to be more rigid 

 towards forces acting east or west than towards forces act- 

 ing north or south. Sir George Darwin suggested that 

 this might be due to the effect of the earth's rotation ; 

 and the problem thus presented has been worked out with 

 great skill by Prof. Love in his Adams prize " On some 

 Problems of Geodynamics." He finds that Hecker's 

 result cannot be explained in terms of the earth's rotation, 

 and proceeds to invoke the direct gravitational influence 

 of the tidal wave in the Atlantic Ocean upon the vertical 

 at any neighbouring locality or the pressure effect of the 

 same wave on the bed of the ocean. The International 

 Association propose to follow up the inquiry by establish- 

 ing new stations in suitably chosen localities, such as 

 Paris, because of its contiguity to the ocean, and a station 

 in the Russian Empire far removed from the Atlantic. 

 The installation of instruments in South Africa, say at 

 Johannesburg, and at some place on the American con- 

 tinent, would also provide results which, in conjunction 

 with the others, might lead to the complete solution of 

 the problem. In carrying out this work the Seismological 

 Association is to be associated with the International 

 Geodetic Association, and funds have been voted for the 

 purpose. 



It has thus been established beyond a doubt that the 

 earth's crust yields appreciably to the tidal action of the 

 moon. The measurement of this yielding demands the 

 use of delicate instruments of a type familiar to seismo- 

 logists, and used by them in the investigation of earth- 

 quake movements. Every large earthquake starts tremors 

 through the earth, and these are caught and recorded at 

 a large number of stations in many parts of the globe. 

 Prof. Milne, working through the Seismological Committee 

 of the British Association, was the first to show the value 

 "f such a seismological survey ; and now it may safely be 

 said that no observatory is complete without a self-record- 

 ing seismograph. Within the last few years groat 

 advances have been made in the perfecting of instruments 

 for registering movements of the ground, and among those 

 who have increased the precision of these instruments 

 Prof. Wiechert and Prince Galitzin are worthy of special 

 mention. Prince Galitzin exhibited at the congress his 

 form of vertical motion seismograph, which utilises the 

 same method of magnetic damning whirl) is characteristic 

 of his horizontal pendulum. The instrument is thus made 

 aperiodic, so that the relative motion of the ground is 

 faithfully recorded. The record is obtained by the action 

 1 11 a delicate galvanometer of induced currents set up in 

 a .oil which moves with the boom of the seismograph 

 to a strong magnetic field. The galvanometer 

 nade aperiodic. Prince Galitzin explained his 

 method of finding the epicentre of an earthquake from 



NO. 2l8o, VOL. 87] 



records obtained at one place on his two horizontal pendu- 

 lums. The interval between the arrival of the first and 

 second phases of the preliminary tremors gives, as Milm 

 and Oldham showed years ago, the distance of the epi- 

 centre. Galitzin has now shown how we may get the 

 azimuth from the first indications of the seismograms 

 obtained with aperiodic horizontal pendulums. The ratio 

 of these first displacements in the east-west and north- 

 south records gives the tangent of the azimuth angle re- 

 ferred to the meridian. There is an ambiguity as to the 

 direction along which the epicentre lies, whether, for 

 example, it is N.N.E. or S.S.W. ; but this ambiguity is 

 quite removed when the vertical component is obtained. 

 The precision of the method depends upon the fact that 

 the instruments are damped accurately so as just to be 

 in the aperiodic state. By a large number of examples 

 Prince Galitzin showed that the estimation of the position 

 of the epicentre in cases in which the method is applic- 

 able was as satisfactory as by the usual method of com- 

 parison of times from two or more stations. 



The interpretation of the times of transmission of earth- 

 quake tremors to places at various distances from the 

 epicentre is one of considerable importance in seismological 

 studies. The curve which shows the relation between 

 distance and time — the hodograph, as many now call it — 

 is not yet known so accurately as one would wish, a fact 

 which was emphasised by Prof. Reid, of Baltimore. 

 Prof. Wiechert's ingenious discussion of inferences to be 

 drawn from the form of this curve has led him to a 

 theory of the constitution of the earth which differs in 

 some respects from other similar theories. To take into 

 account the peculiarities of the hodograph, and especially 

 the flattening of it at mid-stations, which has already 

 engaged the attention of Milne and Oldham, Wiechert 

 finds it necessary to construct the earth of three layers 

 differing in elastic properties, the central core being, in 

 his opinion, made of nickel-iron. A special feature of 

 Wiechert's discussion is the account he takes of the inter- 

 mingling of disturbances which have travelled to the same 

 station, the one directly, the other after one reflection at 

 the surface of the earth. The theory is admittedly 

 approximate, and may require correction as our know- 

 ledge of the hodograph becomes more certain; but the 

 mathematical reasoning by which Wiechert attacks the 

 problem will always have its value apart from the details 

 of any conclusion to which we may be temporarily led. 



Thi :se three lines of research — tides in the earth's crust, 

 determination of epicentres from observations at one 

 station, and inferences as to the structure and physical 

 properties of the inner parts of the earth — probably bulked 

 most largely in the discussions of the congress. But many 

 other points of interest were touched upon, such, for 

 example, as the elastic properties of rocks, for the measure- 

 ment of which Dr. Oddone gave a remarkably simple 

 experiment and calculation based on Hertz's expression for 

 the compression of a sphere during impact. Again, in 

 connection with the cause of the microseismic movements 

 of the earth's surface, Prof. Schuster and Mr. Morris 

 Airey exhibited an instrument designed to count the 

 number of waves which beat on the shore. The increase 

 of pressure due to the passing of a wave was transmitted 

 by means of a hydrostatic and electric arrangement, so as 

 to make the recording pen move always with a slight 

 definite motion in one direction at right angles to the 

 motion of the strip of paper on which the record was 

 taken. After 120 minute and individually imperceptible 

 movements were made, the pen moved back automatic- 

 ally to the original position. The record was therefore a 

 series of diagonal lines the inclination of which to the 

 motion of the paper was greater according as the waves 

 came faster. In the records which were shown the period 

 indicated was about six or seven seconds for each wave. 

 This is one of the commonly recurring periods in micro- 

 seisms. 



The idea of measuring the intensity of an earthquake 

 shock by the overturning of blocks is an old one, and 

 engaged the attention oi Milne and West in the early 

 'eighties. Omori also constructed from these indications 

 a dynamic scale. Prince Galitzin showed how the indirn- 

 tions might be greatly improved by providing the blocks 

 with edge; like the projecting covers of a book. When 



