Interpreting D" seismic structure using synthetic waveforms computed from dynamic models

We have formulated dynamic models of processes hypothesized to give rise to observed seismological structure at the base of the mantle. One dimensional seismic models are determined from thermo-chemical convection calculations from which synthetic waveforms are computed. Of the three scenarios studied, (1) a thermal slab, (2) a thermal slab interacting with simple chemical layer, and (3) a thermal slab interacting with a phase transition, the strength of seismic phases computed from (3) best correlates with the global geography of the observed D " triplication. Seismic structures predicted by (1) or (2) are inconsistent with the global geography and strength of the triplication. Our successful model motivates a new class of 1-D seismic models with two mild gradients and a small discontinuity. To test this hypothesis, we searched for seismic models that both contain these features and fit D " seismic observations. Our preferred model contains a gentle positive gradient initiated 350 km above the CMB and a 1% jump in S-velocity near 200 km. A strong negative gradient begins about 100 km above the CMB, similar to previous studies representing the lower thermal boundary layer. With the reduced first-order discontinuity, the P-wave triplication becomes too small to observe and provides a simple explanation for the observed weakness of the P in proportion to the S triplication. (C) 1998 Elsevier Science B.V. All rights reserved.