Possibility of a low P-wave velocity layer in the outermost core from global SmKS waveforms

A global data set consisting of 1211 SmKS (m ≥ 2) waveforms has been analyzed to investigate the radial seismic velocity structure around the core-mantle boundary (CMB). Band-passed (f = 0.02-0.1 Hz) and stacked waveforms coincide with reflectivity synthetic ones for PREM very well, whereas those for other global models (iasp91, ak135, and SP6) yield disagreements. A preliminary waveform modeling for the outermost core with PREM as initial structure results in a core surface layer with a P-wave velocity of 7.95 km/s at the core top and thickness of 90 km. Based on a 3-D global S-wave velocity model, SmKS phases collected in this study mainly pierce slightly high velocity regions at the base of the mantle, and the predicted differential travel time residuals for S3KS-SKKS are expected to be negative, approximately − 0.5 s for the averages. However, positive S3KS-SKKS residuals, of which average is + 0.3 s, are predominant in the stacked waveforms. Moreover, a waveform modeling for the D″ structure can result in a 30 km thick layer with a 10% S-wave velocity reduction at the mantle bottom (ULVZ), in which waveform fitness for the part of mainly S3KS is improved. On the other hand, the waveform data is not well explained by the lowermost mantle structures with thickness of several hundred kilometers. These suggest that the SmKS data is still affected by the structure of the lowermost several tens kilometers in the mantle that is not sufficiently modeled by a global tomography. Although the thin low S-wave velocity model is not conclusive, the possibility of a low P-wave velocity layer in the outermost core is remained because the waveform fitness for the part of S4KS is improved by the combination of the ULVZ and a 140 km thick layer with a 0.8% P-wave velocity reduction at the core top.