FURTHER STRUCTURAL CONSTRAINTS AND UNCERTAINTIES OF A THIN LATERALLY VARYING ULTRALOW-VELOCITY LAYER AT THE BASE OF THE MANTLE

Constraints and uncertainties are presented for modeling of an ultralo w-velocity zone layer (ULVZ) at the base of Earth's mantle using an SK S wave with small segments of P wave diffraction at the SKS core entry and exit locations, called SPdKS. Source or receiver effects are rule d out as causes far the SPdKS anomalies used to map ULVZ structure, si nce systematic SPdKS -SKS travel time moveout behavior is present in p rofiles of recordings of a given earthquake at many seismographic stat ions and also for many events recorded at one station. The southwest P acific region produces strong variability in observed SPdKSISKS amplit ude ratios (compared to synthetic seismograms), which geographically c orresponds to an anomalous ULVZ region. Accurate determination of abso lute ULVZ thicknesses requires knowledge of, in addition to magnitude of P wave velocity (Vp) reduction in the layer, the magnitude of S wav e velocity (V-S) reduction and density (rho) perturbation (if any). Sy nthetic seismogram experiments demonstrate several key points regardin g uncertainties and constraints in modeling ULVZ structure: (1) thicke r layers (up to 300 km thick) with mild reductions (e.g., -2.5 to -5.0 %) cannot reproduce the anomalous SPdKS behavior seen in the data; (2) for ULVZ layers less than 10 km thick, strong trade-offs exist betwee n discontinuous velocity reductions and linear gradient reductions ove r a thicker zone; (3) uncertainties preclude precise determination of magnitude of delta V-P and delta V-S reductions, as well as the delta V-S:delta V-P ratio; (4) large density increases within the ULVZ (e.g. , up to 60% and more) can efficiently broaden and delay the peak of th e energy that we identify as SPdKS for models with strong velocity red uctions in the layer; (5) models with extreme e reductions in the ULVZ can affect SPdKS waveforms, and dampen spurious ringing energy presen t in Sd waveshapes due to the ULVZ; and (6) the minimum required Vp re duction for the most anomalous data (around -10%) trades off with thin ner ULVZ structures containing larger velocity reductions (with possib le density increases as well).