Wenbo Wu, Jessica C. E. Irving 

Vigorous convection in Earths outer core led to the suggestion that it is chemically homogeneous. However, there is increasing seismic evidence for structural complexities close to the outer cores upper and lower boundaries. Both body waves and normal mode data have been used to estimate a P-wave velocity, Vp, at the top of the outer core (the E layer), which is lower than that in the Preliminary Reference Earth Model. However, these low Vp models do not agree on the form of this velocity anomaly. One reason for this is the difficulty in retrieving and measuring SmKS arrival times. To address this issue, we propose a novel approach using data from seismic arrays to iteratively measure SmKS-SKKS differential travel times. This approach extracts individual SmKS signal from mixed waveforms of the SmKS series, allowing us to reliably measure differential travel times. We successfully use this method to measure SmKS time delays from earthquakes in the Fiji-Tonga and Vanuatu subduction zones. SmKS time delays are measured by waveform cross-correlation (CC) between SmKS and SKKS and the CC coefficient allows us to access measurement quality. We also apply this iterative scheme to synthetic SmKS seismograms to investigate the 3D mantle structures effects. The mantle structure corrections are not negligible for our data and neglecting them could bias the Vp estimation of uppermost outer core. After mantle structure corrections, we can still see substantial time delays of S3KS, S4KS and S5KS, supporting a low Vp at the top of Earths outer core.