THE EFFECT OF A LOW-VELOCITY INTERNAL STRATUM ON SURFACE ELASTIC WAVES

This paper originates in a remark by Professor B. Gutenberg that” the properties of surface waves under the assumption that the velocity of body waves has a minimum at a certain depth have not been investigated yet for the solid earth, though such a minimum seems to exist”. Actually, one aspect of the problem was discussed in 1929; rather contrary to expectation, it was shown that in a representative case no gap will exist in the spectrum of Love waves. The effect of Rayleigh wave propagation is likely to be small, since Rayleigh waves can exist in a homogeneous medium. Love waves, however, require a heterogeneous medium for their existence, and it is therefore with Love waves that critical conditions may be expected. The present paper discusses the effect of a low-velocity internal layer, of the type envisaged by Gutenberg and Richter, on the surface amplitude of Love waves, and it is shown to be relatively unimportant. The problem of a high-velocity internal layer is also discussed; Love waves will not exist for waves longer than a certain critical wave-length depending on the parameters of the system. As a limiting case of the three-media systems considered, it is shown that for sufficiently short wave-lengths Love waves can be propagated in a two-layer plate; the velocity is greater than the velocity of distortional waves in either of the two layers.