A lower mantle S‐wave triplication and the shear velocity structure of D″

Abstract

Summary. A lower mantle S‐wave triplication detected with short‐ and long‐period WWSSN and CSN recordings indicates a substantial shear velocity discontinuity near 280 km above the core–mantle boundary. The triplication can be observed in rotated SH seismograms from intermediate and deep focus events throughout the distance range from 70° to 95°. Three distinct source region–receiver array combinations that have been investigated in detail demonstrate consistent travel time and relative amplitude behaviour of the triplication, with slight systematic shifts in the triplication indicating up to 40 km variations in the depth of the discontinuity. Modelling of the observations with synthetic seismograms produced with the Cagniard de Hoop and reflectivity methods constrains the shear velocity increase to be 235 ± 0.25 per cent, comparable to upper mantle discontinuities. Short‐period observations indicate that the velocity increase may be a sharp first‐order discontinuity, or may extend over a transition zone no more than 50 km thick. The shear velocity gradient below the discontinuity, within the D″ layer, is not well‐constrained by the SH data, but slightly positive or near zero velocity gradients are consistent with the long‐period amplitude ratios of ScSH/SH. Copyright © 1983, Wiley Blackwell. All rights reserved