Seismometry and Gěite

Abstract

UNDER the above heading Prof. J. Milne contributed an interesting article to NATURE of April 9, p. 538, on which I wish to offer some remarks. Prof. Milne seems hardly to realise the significance of the enormous pressures to which the earth's deep-seated material is presumably exposed. One of his objections to the hypothesis of an iron core seems to be that the wave velocities for an infinite isotropic medium of the density and elasticity of iron do not accord with the velocities of earthquake waves. This objection, however, is not conclusive. In an infinite isotropic medium there are two purely elastic wave velocities, v₁ and v₂, given by the equations where ρ is the density, m and n Thomson and Tait's two elastic constants. On the ordinary theory, n/m may possess any value consistent with Poisson's ratio γ,or (m-n)/2m, lying between 0 and 0.5. Six years ago I showed (Phil. Mag., March, 1897, p. 199) that observed seismic wave velocities can be accounted for by elastic waves without postulating any abnormal value for Young's modulus—the modulus to which Prof. Milne repeatedly refers. For instance, we get values of 12.5 and 2.5 kilometres per second respectively for v₁ and v₂ in a medium of density 5.5 with a Young's modulus of only 10⁹ grammes weight per sq. cm., if we suppose n/m = 1/24, or γ = 0.48 approximately; and the same results follow if we increase density and elastic constants in the same proportion.