Reflection properties of the core‐mantle boundary from global stacks of PcP and ScP

Abstract

Seismic phases that reflect off the core‐mantle boundary (CMB) are sensitive to the velocity and density contrasts between the base of the mantle and the core. We measure the amplitudes of CMB reflections to seek effects of large velocity reductions and possible density increases in proposed thin basal layers (ultralow‐velocity zones, or ULVZs). We construct globally averaged envelope stacks of PcP and ScP in 3°‐wide epicentral distance bins, correcting for propagation and source effects. We measure the amplitude ratios PcP/P and ScP/P and compare them with the predicted range dependence for preliminary reference Earth model (PREM) and several proposed ULVZ models. The amplitude ratios are not compatible with ULVZs containing 30% S wave velocity reductions or with a 3‐km‐wide core‐mantle transition zone. The PcP data cannot distinguish between PREM and models with 10% reductions in both P wave velocity (VP) and S wave velocity (VS) and up to a 20% increase in density. The ScP data also match PREM closely but contain greater uncertainty from the correction for mantle attenuation. In well‐sampled regions, lateral variations of reflection amplitudes mapped to bounce point locations are consistent with previous detections and nondetections of ULVZs. These results suggest that extreme velocity reductions near the CMB are not global features and that regions with large velocity reductions do not result from global phenomena such as phase changes in an isochemical mantle but rather from local thermal or compositional anomalies.