Wave Velocities in the Outer Part of the Earth's Mantle

Abstract

Hitherto, wave velocities V in the earth's interior have been calculated from the travel times t of earthquake waves in shallow shocks computed as a function of the epicentral distances Δ. This procedure fails if at any depth the velocity V decreases with depth at a rate in excess of dV/dr = V/r, where r is distance from the centre. In this case a portion of the travel-time curve is formed by the arrival of diffracted waves; if these are misinterpreted as direct waves, incorrect velocities are found not only for the low-velocity layer, but also (with decreasing errors) for all deeper layers. Moreover, discontinuities may be inferred when none actually exists.