Array-Based Iterative Measurements of SmKS Travel Times and Their Constraints on Outermost Core Structure

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