The shadow of the Earth's core

Abstract

There is very good agreement between the observed decrease in amplitudes of longitudinal waves diffracted at the boundary of the earth's core and corresponding theoretical results. Especially, no rapid decrease of amplitudes at the beginning of the shadow zone is to be expected. Theory and observations show that the amplitudes of diffracted short-period P waves decrease faster with distance in the shadow zone than those of longer waves. At epicentral distances of over 110° short-period diffracted P waves emerge gradually and their beginning can rarely be ascertained. Corresponding long-period waves arrive within the limits of error at the time calculated on the assumption of a straight-line travel-time curve. Amplitudes of waves diffracted from a caustic decrease rapidly with distance; this usually limits the range of their observation to roughly 10°. On the contrary, amplitudes of P waves diffracted at the core may be visible on records of a great earthquake at distances of 103° to 180°. Observations for diffracted S waves cover a much smaller range of distances, since S waves appear in a portion of seismograms which is disturbed by earlier motion. Otherwise, observations indicate similar behavior for diffracted P and S waves.

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