Structure of the Earth
Earthquakes have yielded much of our present knowledge of the earth's internal structure. Earthquake waves traverse every part of the earth's interior, and from the study of travel times one can reconstruct the wave paths and calculate the velocity of travel at each point. This study has led to the fundamental discoveries that the earth is an elastic body and that its elastic properties are to a high degree symmetrical about the centre.
In 1906 Oldham showed that the earth has a central core with radius 2,200 miles (compared with 4,000 miles for the whole earth). Both P and S waves travel in the surrounding mantle, which makes up about five-sixths of the earth's volume, but no S waves have been found to travel in the core. Thus the core appears to be fluid; it is commonly supposed to be made of molten iron and nickel. At the very centre there is a small inner core with radius 800 miles. This was discovered in 1936 by the Danish seismologist, Miss Lehmann, from studies of two New Zealand earthquakes recorded in Europe. The velocity of P waves appears to increase suddenly on entering the inner core, and the New Zealander, K. E. Bullen, has recently found evidence that the inner core may be solid. Some of the types of path followed by earthquake waves in the earth's interior are sketched in diag. 5.
The symmetry of the mantle and core does not extend to the thin outer crust. The patchwork of continent and ocean that we see at the surface connotes important differences in the crust itself. Beneath the oceans the solid crust has a thickness of 3–6 miles, while the continental crust is on an average 20 miles thick. The crust-mantle boundary is thus about 15 miles deeper under the continents; this boundary is called the Mohorovicic discontinuity after the Croatian seismologist who discovered it in 1909.
Although the crust makes up only about 1 per cent of the earth's volume it naturally holds a special interest for mankind. Here, too, our chief knowledge has come from the study of earthquake waves, though natural earthquakes have been supplemented by artificial ones generated by means of gelignite explosions.
Part of the energy of large shallow earthquakes becomes trapped within the crust and travels in the form of guided waves, known as Love waves and Rayleigh waves. The character of these waves is influenced by the crustal wave guide, and they can be used to find the thickness of the crust through which they have travelled. New Zealand seismologists, applying this technique to earthquakes recorded at Hallett Station and Scott Base, have proved that the Antarctic ice cap is underlain by continental land in eastern Antarctica, whereas western Antarctica is not fully continental. Similar studies have shown very recently that New Zealand itself has a continental crust — a discovery of much scientific interest in view of the relatively small area of New Zealand.