Further structural constraints and uncertainties of a thin laterally varying ultralow-velocity layer at the base of the mantle

Abstract

Constraints and uncertainties are presented for modeling of an ultralow-velocity zone layer (ULVZ) at the base of Earth's mantle using an SKS wave with small segments of P wave diffraction at the SKS core entry and exit locations, called SP_dKS. Source or receiver effects are ruled out as causes for the SP_dKS anomalies used to map ULVZ structure, since systematic SP_dKS-SKS travel time moveout behavior is present in profiles of recordings of a given earthquake at many seismographic stations and also for many events recorded at one station. The southwest Pacific region produces strong variability in observed SP_dKS/SKS amplitude ratios (compared to synthetic seismograms), which geographically corresponds to an anomalous ULVZ region. Accurate determination of absolute ULVZ thicknesses requires knowledge of, in addition to magnitude of P wave velocity (V_p) reduction in the layer, the magnitude of S wave velocity (V_S) reduction and density (ρ) perturbation (if any). Synthetic seismogram experiments demonstrate several key points regarding uncertainties and constraints in modeling ULVZ structure: (1) thicker layers (up to 300 km thick) with mild reductions (e.g., −2.5 to −5.0%) cannot reproduce the anomalous SP_dKS behavior seen in the data; (2) for ULVZ layers less than 10 km thick, strong trade-offs exist between discontinuous velocity reductions and linear gradient reductions over a thicker zone; (3) uncertainties preclude precise determination of magnitude of δV_P and δV_S reductions, as well as the δV_S:δV_P ratio; (4) large density increases within the ULVZ (e.g., up to 60% and more) can efficiently broaden and delay the peak of the energy that we identify as SP_dKS for models with strong velocity reductions in the layer; (5) models with extreme Q reductions in the ULVZ can affect SP_dKS waveforms, and dampen spurious ringing energy present in Sd waveshapes due to the ULVZ; and (6) the minimum required V_p reduction for the most anomalous data (around −10%) trades off with thinner ULVZ structures containing larger velocity reductions (with possible density increases as well).