EARTHQUAKES AND EARTHQUAKE OBSERVERS. DR P. MARSHALL'S VIEWS.

Otago Witness, Issue 2719, 25 April 1906, Page 30

EARTHQUAKES AND EARTHQUAKE OBSERVERS.

DR P. MARSHALL'S VIEWS.

. INTERVIEW WITH A SEISMOLOGIST. '. CFkom Oub Owtn Correspondent.) WELLINGTON, April 20. This evening I called upon Mr Hogben ; at his residence in Tinakori road to sco his records of the fcsan Francisco earthquake, and to have a " yarn" of a few minutes' dirration about earthquakes in general ; ' but so fascinating did the conversation become that instead of a few minutes I .stayed two hoars. I find that the interview with him as published in the local papers, -^.-liioH I telegraphed yon <;<ai-lj-o-r-i"> the •evening, is inaccurate in several particulars, and I had better, therefore, give you the more accurate details. I found Mr Ho?l:en in his study surrounded by yards and yards of deve'oped " tape," and various volmvus and pamphlets on earthquake, and Titeraturo in var.ous languages. Indeed, so great was the mass of records, calculations, and literature that Mrs Hogben humorously remarked that I might he excused if I imagined that there had been an earthquake in the room. I first of all drew Mr Hogben's attention to the cabled suggestion by Professor Milne, to iho effect that the earthquake was probably caused by the earth's swinging a little off its axis, and the strain of the struggle to get back brpaking the earth's crust. Mr Hogben evidently • did not Avish in any way to discredit the valuable work that Mr Milne has done, but in regard to tlis latest theory he coulrl not help remarking that itwas.to .aay the least of it, /"bizarre." This led to a "general conversation, about earthquakes, and though Mr Hogben did not say so, I may perhai.-s be permitted to remark that Professor Milne is not quite the great authority that s-ome people here, and even in England, imagine him to be. He is undoubtedly a recognised seismolcglcal statistician. Milne is now in the Isle of Wight, and is secretary to the British Association's Siesmolo^ical Committee; his work being largely statistical. The reports compiled are, Mr Hogben -says, very valuable indeed. Mr Hogben sends his New Zealand records to him, and he collates all the records ho receives from various quarters of the globe. In answer to a question about the statement of the Rev. Mr Fairclough (telegraphed you last night) that the transference of material by rivers from the land surface to the ocean bed was not now generally considered to be "so great a factor in causing earthquake's, Mr Hogben pointed out that Dutton, the best United States seismologist, in his latest work gives a contrary opinion. Indeed, he goes so far as to say that the transfer of sediments is the only obvious and plainly visible cause which has thus far been -suggested as a source of these cumulative stresses, which ultimately become resistless and lead to the -collapse which generates the earthquake. The statement, as Mr Hogben pointed out, may, however, be regarded as too sweeping. Indeed, it is so regarded by the German, the British, and the Japanese authorities. As Mr Hogben has himself pointed out, the cooling of the 'earth is" another important factor. Observations in deep mines show that the temperature of the earth increases with the depth below the surface, and at the depth of a few miles the temperature must be very high, as, the heat is sufficient to bring what are, on that account, called igneous rock to the melting point. The average rate increase near the surface is known to be about ldeg. Fahr. for every 51ft of descent, but that rate cannot con- . tinue to all depths, for at the depth of J 200 to 400 miles the temperature would be ' equal to the temperature of the sun's sur- ' face. The probability is that the rate of increase of temperature diminishes with the depth, and that about 30 miles below the surface the rocks are in a state of potential liquidity. That is, they are so hot that they would become liquid were it not for the pressure to which they are subjected. At some depth below that-r-100 1 miles below the surface according to Lord Kelvin, but much less according to others — the whole mass is, whether liquid or solid, probably very nearly at the proper melting temperature for the pressure, so that the : state of the matter there cannot affect the i temperature in the strata above. These 1 upper strata form what is probably called 1 the crust or lithosphere, the inner portion being the nucleus or centrosphere. Heat will continually pass from the inI terior to the surface, where it ] will be dissipated into space. The temperature of the rocks near the surface, being the temperature of space modified by the heat from the sun's rays, is prac- , tieally constant, so that those rocks will not contract. All other layers of the j crust will cool and contract at rates varying with , the depth. The volume of the layers in which cooling is greatest will, after 'contraction, be too small to fill -the space into which the layers fall, and they j will therefore be pressed out by the weight lof the rooks abovs them. But the upper strata, which cools less quickly, will .be too lar^e to fill the space into which they fall, and their surface will accordingly be crumpled. In regard to "loading," Mr Hogben says another cause of folding is the transference of material from the land surface to the bottom of the sea by the agency of rivers. The pressure on the strata underlying the land will be thereby relieved, and tlia pressure fin the floor of the ocean will be increased. There will thus be a folding of the strata near the junction of the land surface and the ocean bed which may continue until fracture takes place, the strata on the land side moving up and those on the ocean side moving down. The displacement of the strata will form a normal fault. After the fracture the two faces of the strata will continue to slip and to slide, one over the other, so as to increase the amount of -displacement. The slipping may go on very gradually, but from time to time sudden slips will probably occur, and these will produce earthquakes. Coming back to the San Francisco earthquake, Mr Hogben showed me the developed tapes. These prove that the first ( waves reached Wellington at Ohr 56min 12seo New Zealand time on the morning of the 19th April. There are two distinct , shocks, tho waves of the second one reach- j 1 ing here at Ohr 59min. . There was thus ; about three minutes between the two. ', These shocks, known as the preliminary tremors, would be sufficiently severe to wake sleepers and came great alarm, but they would not last sufficiently long to do much damage. It is really the length of time the " swing " continues that does the damage. The long" waves, representing the movement that takes place after the fracture, which aie less rapid, but very much lugger, reached here lit lmin 40s.ee,

or nearly 43min after the second preliminary shock. Thes3 figures have not yet been corrected by Mr Hogben, but they will be found to' be" not; more than a very few seconds out. That interval indicates fairly well the distance of the observation from th^g origin of the quake, and it corresponds with San Francisco or a little on this side' of it. The exact spot may be in the ocean, but it cannot be exactly stated until the observations from, four or five places have been obtained. The long waves show a speed of over two miles per second, and the small or preliminary waves showed a speed of between nine and ten miles per second, and go to prove the great intensity of the earihtjuake. A. liis^ t-elool fey suet as iha.*^ is generally an indication of high intensity, though the elasticity of the rocks through' which the waves travel must also be considered, some rocks being more elastic than others. There will probably be several slight quakes for some days at San Francisco, but, as a rule, such subquent shocks do no damage comparable ,to those resulting from the actual fracture. They are caused simply by the rocks adjusting themselves. It is interesting to note that the New Zealand East Coast earthquake of 9ti August, 1904, and also the Cheviot earthquake were both recorded at two or three stations in Great Britain. . Mr Hogben speaks very highly- <jf the» work of two -Japanese observers, -Profeseoicsv Omori and^ Qinori, he -says,"" is before anybody else jn 1 the science. These twio observers carefully examined the theory of Milne that earthquake waves came through the carth — i.-c.; along a "cord" of the eartfcr-inste'aS of round the earth, at a depth of about 20 miles from the surface, which/" for'-soientifio^purposes, may be called just below the surface. Their investigations «o to prove that there are no grounds >. for the theory.. Mr Hogben has also examined this idea, and has come to the same conclusion as the Japanese scientists..

y The following observations, kindly supplied by Dr P. Marshall, D.Sc.,~"will doubtless prove interesting at the present jurioture: — " The regions of the earth's crust that are most frequently shaken by earthquakes are all situated on steep slopes of the. earth's crust, extending from the summit of a mountain range to the bottom of oceanic depths. San Francisco is situated in the middle of one of the most pronounced of these slopes, and has been known to be subject to earthquakes since its foundation. These steep slopes are eminently unstable regions of the earth's crust, for they owe their formation to the immense lateral pressure resulting from the efforts of the solid crust of the earth to adjust itself to a cooling, apd therefore shrinking, nucleus. From time to time - the strains thu£-feet up in *he crust overcome the resistance- of the.solid xoeks. A . fracture is formed, and the rocks on one • side of it slide past those on the other.' until .the strain is relieved. Usually tho relief is afforded by, sudden jumps, and ■, • the movement is a series of jerks, each of , which generates, an earthquake shock. The s first movement is generally me largest, ana" the shock it causes is the worst. As a rule such earthquakes are not associated with volcanic eruptions. If th>y arel they - precede the eruptions, as in the case of the Guatemala earthquake before the erup- , tion of Mont Pelee. It is extremely unlikely that there is any relation between the eruption of Vesuvius, or the' earthquake at Formosa, and tho disaster at San Francisco. Whether the fracture of the rocks has caused any subsidence at the surface in this case is not clear from the cables, for the submergence of some portions of the water front might as well be due to tidal waves, which are always . generated by earthquakes in coastal districts. \ The whole subject of the generation and propagation of earthquakes has lately been very ably summarised by Mr Hogben, a well-known authority, for the recentlypublished" New Zealand Geography.