EARTH'S INTERIOR

Evening Post, Volume CXXVIII, Issue 5, 6 July 1939, Page 11

EARTH'S INTERIOR

FORM OF STRUCTURE

WHAT EARTHQUAKES TELL

MUCH TO BE LEARNED

Most people look on earthquakes as a distinct menace to their peace of mjnd, and perhaps to the safety of their lives and property. But, by Scientists earthquakes are regarded from another point of view. The scientist sees in an earthquake a tool which may be utilised for investigating the inner structure of the earth. In fact, it is almost the only tool he has at present, for investigating this problem,, He is under no illusions about his knowledge or lack of knowledge of earthquakes* and makes no claims about being able now or in the near: future to predict them with accuracy. This was made clear, by Dr. Lyndon Bastings recently when he addressed members of the Royal Society on "Earthquakes and Earthquake Prediction."

"It can be said without any hesitation," he remarked, "that a complete and accurate .prediction of an earthquake has never been made arid probably never will. For a description of; an earthquake, either.past or future, to *be accurate, it must specify the locality, the time, and the magnitude of ! the initial disturbance. So'many incidental factors are capable of influencing both the time and the magnitude Of .the shock, that there is very little hope of sufficient knowledge ever being assembled to deal adequately with, all of them. It will be much, more profitable if an intensive study is made of the signs of. growing strain in the rocky crust, in order to be able to foresee which localities are most liable to a visitation in the future, and so to be able to lay the foundations of a sound policy in regard to -building safety commensurate with the indicated risks." A CORE OF IRON. Pealing with the inner structure of the earth as revealed by earthquakes, Dr. Bastings said- that prior to the development of modern seismology, gravitational and magnetic evidence indicated that the. deep interior of the earth probably contained a large ma§s of metallic iron. The study of records of large earthquakes had not only confirmed this idea, but had made it possible to define the boundary of this iron core with some precision. It appears to be symmetrically situated at the centre of the earth, and to have a radius of about 2200 miles, or a little over onehalf of the earth's radius. Its boundary is fairly sharply marked off from the mantle outside, and, in consequence, earthquake waves are reflected sharply when they meet it. On returning to the surface again these waves record their story of the encounter in terms which are-capable of interpretation.

A controversy has arisen as to whether this iron core is solid or liquid. Most of the evidence in favour of a j solid core has been derived from earthquakes in the New; Zealand region, | the most notable of these being the Buller earthquake of 1929. But " the evidence is extremely difficult to unravel, and has not appealed with full i conviction to all competent authori-■ ties. It was unlikely, that the controversy .would be settled until another1 majQr earthquake'occurs in a region, and under circumstances favourable to the detection of the crucial movements upon which the evidence for solidity rests. , ~:., : MASS NOT UNIFORM. ' Recent progress in instrumental accuracy, taken in conjunction with developments in theory, have made it' possible to identify one of more other i breaks in the continuity! ofi the structure deep down in the earth, i The most important of these breaks, next to that at the core boundary, is of a much less striking kind, and probably represents the passage from one type of rock-material to another, with a small] but discrete change in density. This | transition is now believed to be situated j at about 300 miles below the earth's surface. Several other breaks in the continuity of the 'deep interior have been announced by various investigators; but none of themhaye received the general recognition accorded.'to these two. v; ■ ::'-f" '"<:"-' ■■■-' ■'■ .■■'■■■■"■ " '■

: Work on the superficial structure, both in the New Zealand region and also in other prominent earthquake regions of the world, is making steady progress at the present time, and it should be possible before very long to be able to state with some accuracy the nature, extent, and thickness of the two or three upper layers, which are believed to compose the fifty or so miles of the earth's crust. Evidence is also accumulating to indicate that at greater depths than this, the symmetry of the earth's interior is marred somewhat by the existence of a slight structural difference between the Pacific basin and the rest of the world. How marked and how deep this difference may be it is,too early yet to say, but developments that are expected shortly may throw valuable light on the difficult question of the origin of the Pacific Ocean. The lecture, which was followed with great interest and which gave rise to a number of questions, was illustrated by a number of slides and diagrams, many of which vv;ere reproductions of the seismograms of various New Zealand earthquakes as recorded in distant parts of the world.