Seismological Evidence for Heterogeneity of the Mantle

Abstract

The latest results concerning the depth variation of Q, P- and S-wave velocities obtained by various seismological methods are reviewed with special emphasis on the regional variation. The body-wave travel time and the slope, dT/dΔ, have been inverted to obtain P- and S-wave velocity distributions within the mantle. The P velocity is now believed to be known with an accuracy of ±1% in the mantle below 1000km. In the upper mantle, a considerable difference has been found among different regions. The long-period surface waves have been used to reveal the regionality of the upper mantle. Based upon the surface wave and body-wave results, it is now agreed that the mantle beneath shields has much higher seismic velocities than those beneath ocean and tectonically active regions. At such tectonically active regions as Japan, basin and range province of the western U. S., and the Pacific ocean side of South America, the upper mantle velocities have been found to be significantly lower than the average. Teleseismic P waves are delayed by 1.0 to 1.5 sec at tectonically active regions as compared with shields. The existence of the low-velocity zone for S waves in the sub-oceanic mantle is almost certain. The shape of the low-velocity zone varies markedly beneath continents. The distribution of Q with depth is not known in detail. However, it is almost certain that the value of Q is higher by more than one order of magnitude in the mantle below 1000km than in the mantle above. The average values of Qα (Q for P-waves) and Qβ (Q for S-waves) for the upper mantle range from 100 to 300. Horizontal heterogeneities as large as a few per cent in velocity and one order of magnitude in Q have been found beneath mid-oceanic ridges and island arcs. However, the uniqueness of the structure derived is still questionable.

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