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

Abstract

AbstractVigorous 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 wave velocity, , at the top of the outer core (the layer), which is lower than that in the Preliminary Reference Earth Model. However, these low models do not agree on the form of this velocity anomaly. One reason for this is the difficulty in retrieving and measuring arrival times. To address this issue, we propose a novel approach using data from seismic arrays to iteratively measure ‐ differential travel times. This approach extracts individual signal from mixed waveforms of the series, allowing us to reliably measure differential travel times. We successfully use this method to measure time delays from earthquakes in the Fiji‐Tonga and Vanuatu subduction zones. time delays are measured by waveform cross correlation between and , and the cross‐correlation coefficient allows us to access measurement quality. We also apply this iterative scheme to synthetic 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 estimation of uppermost outer core. After mantle structure corrections, we can still see substantial time delays of , , and , supporting a low at the top of Earth's outer core.