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N A TURF. 



[April 23, i< 



niucli to popularise seismology is the fact that u very 

 large eartliqualce originating in any one part of the world 

 may be recorded in any other portion of the same. This 

 means that the opportunity for carrying on seismological 

 research is not a monopoly enjoyed only by those who 

 reside in earthquake countries. Although only a few 

 persons in Great Britain have been privileged to feel one 

 of its home-made tremors, every one of its inhabitants is 

 very many times per year moved by earthquakes. Back 

 and forth motion of the ground is performed too slowly 

 for us to feel, while, if there is a movement like the swell 

 upon an ocean, the undulations are too long and flat for 

 us to see. 



Waves start out from their epicentral area, which is a 

 district that has been fissured and shattered by the forma- 

 tion or extension of large faults in all directions. Observa- 

 tion, however, shows that these waves are propagated 

 farthest in one particular direction. For example, the 

 thief movement following the San Francisco earthquake, 

 which originated from fault lines running parallel to the 

 coast of California, was much more marked in countries 

 lying to the east or west of California than in countries 

 lying towards the south. England and Japan obtained 

 large records of the disturbance, while in Argentina the 

 records were extremely small. In the case of the Jamaica 

 earthquake, where the lines of origin ran east and west, 

 the phenomenon was reversed. Toronto received a large 

 quantity of motion, and England a very little. Another 

 peculiarity of this phase of earthquake motion is that it 

 may be propagated in one direction round the world to a 

 greater distance than in an opposite direction. The sugges- 

 tion is that the initial impulse was delivered in the direc- 

 tion towards which motion was propagated farthest. If 

 for illustration we assume that the slip on a fault line 

 has been downwards towards the east, then the motion 

 would travel towards the east farther than it would 

 towards the west. That which happens corresponds to 

 >vhat we see if we dip the blade of a spade in water and 

 suddenly push the blade in some particular direction. The 

 water waves thus created travel farthest in the direction 

 of the impulse. 



Another curious phenomenon connected with the large 

 waves of certain earthquakes is that thev can pass their 

 ■equatorial or quadrantal region unobserved. They may be 

 very marked for looo miles round their origin, and record- 

 able, but much reduced in size, about their antipodes, but 

 not recordable in between. For example, an earthquake 

 originating near New Zealand mav be recorded in that 

 country, but not in India, Egypt, West Asia, or east of 

 Kurope, though in Britain it may make itself evident by the- 

 thickening of a photographic trace. The phenomenon may 

 be compared to a water wave running down an exp.inding 

 estuary. At the mouth of such an estuary it may have 

 become so flat that it is no longer recognisable. Should 

 it, however, run up a second estuary, we can imagine 

 concentration taking place, so that near the top of the 

 second estuary it would eventually become instrumentally 

 recordable. In these antipodean survivors we see the final 

 efforts of a dying earthquake. It is only occasionally 

 that the precursors and the followers of these large waves 

 'have sufficient energy to reach their antipodes. They die 

 I'll toute. The former, notwithstanding their comparative 

 "feebleness, because they throw considerable light upon the 

 internal constitution of our earth, are the most interesting 

 feature in a seismogram. They are of two kinds, a first 

 phase and a second phase. These are usually regarded 

 as compressional and distortional modes of wave propaga- 

 tion. The large waves arc probably quasi-elastic gravita- 

 tional waves, something like an ocean swell, which travel 

 round the world with a constant velocity of about t. km. 

 per sec, causing continental surfaces to rise and fall like 

 huge rafts upon a heaving ocean. The precursors behave 

 quite differently. Phase i. may commence with a velocity 

 of T, km. or 4 km. per sec, but as the length of the wave'- 

 path increases this quickly rises to 10 km., and thence 

 to a maximum of 12 km. per sec. These paths are 

 assumed to be along chords, and so long as these chords 

 do not lie at a depth greater than twenty or thirty miles, 

 the speeds are such as we should expect to find in materials 

 like those composing the outer surface of our earth, 

 rhesp waves, therefore, indicate a thickness for the earth's 



N"o. 2no<S, VOL. 77] 



crust comparaD:e u) thicknesses which have been arrived 

 at by other lines of argument. The rapid approximation 

 to uniform speed suggests that below a depth of twenty 

 or thirty miles we enter a nucleus which is very rigid and 

 fairly homogeneous. The second phase waves, up to a 

 distance of 120° from their origin, have a speed of about 

 6 km. per sec. For longer paths, Mr. R. D. Oldham 

 points out that their velocity is apparently suddenly re- 

 duced. He seeks for an explanation of this by postulating 

 the existence of a central core in the earth where waves 

 are retarded and refracted, with the result that the wave- 

 paths no longer follow chords. These waves may, there- 

 fore, emerge on the surface of the earth after having 

 passed relatively to their starting point on the farther side 

 of its centre. Whether we do or do not accept this central 

 core, it is clear that the new seismology has added in a 

 very marked manner to the knowledge we formerly 

 possessed respecting the interior of the globe upon which 

 we live. Our ideas respecting its homogeneity and its 

 great rigidity have been changed by seismological investi- 

 gations. 



When large earth waves sweep round the world, it is 

 found that at particular stations magnetic and electro- 

 meter needles have been disturbed. Magnetometers, when 

 installed at Toronto, do not appear to have responded t. 

 the slow undulations of the earth's surface, while the sann 

 instruments, after being removed to .'\gincourt, only tin 

 miles distant, are now affected. The inference from thi^ 

 and observations in other parts of the world is that the 

 movements, rather than being caused mechanically, may 

 be due to the disturbance of some adjacent magnetic 

 magma. If this is the case, then at particular stations 

 where the movements due to teleseisms correspond with 

 unusual disturbance of magnetic needles, inasmuch as a 

 magnetic magma is denser than common rock, at these 

 stations the value for g should be higher than that which 

 would be anticipated. For certain stations this appears to 

 be the case. 



Another series of investigations which may widen our 

 knowledge respecting conditions and operations beneath 

 our feet are based upon the light effects which have been 

 so frequently observed at the time of large earthquakes. 

 .Accounts of luminosity in the heavens and on hills as 

 accompaniments to large earthquakes are common. \i 

 the time of the Valparaiso earthquake, August 17, iqofi, 

 the attention of very many people was attracted to lights 

 which appeared upon the hills. Captain Taylor, of the 

 R.M.S. Orissa, compared these to chain lightning, which 

 extended as far as the eye could reach, .^n acquaintance 

 of mine, Mr. G. E. Naylor, of Valparaiso, told me that 

 he saw the lights repeatedly, and they took place immedi- 

 ately before a shock, there being only a fraction of a 

 second of time between the two. He described them a", 

 having a bluish tinge ; to others, however, they appeared 

 yellowish. An ordinary explanation for these appearance:, 

 is that they are due to the rubbing together of rock surfaces 

 or the discharge of frictionally produced electricity. These 

 observations suggest that with a megaseismic collapse, not 

 only do we get mechanical disturbances which pass through 

 and over the surface of the world, but that part of the 

 initial energy at the origin is converted into some other 

 form of energy, which possibly may find a response at 

 very distant places. This latter transmission would, how- 

 ever, take place with a velocity comparable with that ol 

 light. If anything of this sort has a real existence, seismo- 

 logists may hope to record earthquakes at the moment 

 they take place. This consideration, and the observation 

 that from time to time a quarry in the Isle of Wight, 

 known as Pan Chalk Pit, appeared to me to be luminous, 

 suggested the possibility of hypogenic activities giving 

 evidence of their existence in the form of light. Pan Pit 

 faces north, and in winter it is not reached by the sun. 

 Its glowings apparently rise and fall in intensity, and are 

 most noticeable after a dull, damp day. The experiments 

 I made were as follows : — at the end of a chamber twenty 

 yards from the mouth of a tunnel driven into the chalk, a 

 hole about 2 feet square was excavated. Into this a box 

 with a light-proof door was cemented. The back of the 

 box, which touched the chalk, was made of zinc In the 

 zinc three holes of different sizes were m.ade along a 

 vertical line. A cylindrical drum, covered with bromide 



