Shear velocities at the base of the mantle from observations ofSandScS

Abstract

The observed amplitude ratios of transversely polarized shear waves ScSH/SH exhibit a minimum at a distance of about 68°. Synthetic seismograms computed for a Jeffreys-Bullen model and for models with negative linear velocity gradients at the base of the mantle fail to explain this feature. Various positive linear velocity gradients above the core-mantle boundary explain the amplitude ratio minimum as well as an apparent difference in arrival times of the transversely and radially polarized core reflections ScS. Good agreement between the observed amplitude ratios and the computed ones cannot be achieved without assigning low Q values to the lower mantle or a small shear velocity value to the outer core. At epicentral distances greater than 70° a substantial part of the energy recorded as a horizontally polarized ScS wave bottoms at various depths in the high-velocity region rather than at the core-mantle interface. These precursory arrivals are in phase with the core reflections on the transverse component and out of phase with them on the radial component. This difference causes an apparent time differential by which the transversely polarized ScS wave seems to arrive slightly earlier than the radially polarized ScS wave. High-velocity regions between 40 and 70 km thick containing increases from 0.3 to 0.5 km/sec above the velocity of a Jeffreys velocity model yield the best explanation to the combined amplitude and differential time data.