CCMOC: A New View of the Earth's Outer Core Through the Global Coda Correlation Wavefield

Increasing seismic evidence has accumulated, suggesting that the Earth’s outer core consists of distinct layers of low P-wave velocities relative to the Preliminary Reference Earth Model (PREM) in the top and bottom of the liquid core. Seismically detected low velocity in the outer core could be linked with the stratification, essential for understanding the geodynamo and thermochemical evolution of the liquid core. However, a consistent globally-averaged radial structure of the outer core has not been obtained due to the incomplete coverage of sampling body waves. To remedy this problem, we explore the seismic structure of Earth's outer core by employing a new theoretical and observational concept termed coda correlation wavefield. We construct the global correlogram in the 15-50 sec period range by stacking cross-correlations of the long-duration coda waves from the selected ten large earthquakes. We then assemble a dataset of prominent correlation features from the global correlogram that are sensitive to the outer core. The waveforms of these features are fit by computing synthetic correlograms through various outer core models. The obtained optimal model displays P-wave velocities in both the outer core's top and bottom, consistent with Coda Correlation Reference Earth Model (CCREM) and reduced relative to PREM. The low seismic speeds in the top of the outer core could likely imply the formation of a thermal and/or compositional stratification.