CCMOC: A new view of the Earth's outer core through the global coda correlation wavefield

Abstract

Increasing seismic evidence has accumulated, suggesting that the Earth's outer core consists of distinct zones of low P-wave velocities in the top and bottom regions relative to the Preliminary Reference Earth Model (PREM). Seismically detected low velocities 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 s 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 P-wave velocity is ∼1% lower in the core's top than that in PREM, and the slow anomaly gradually approaches zero at about 800 km below the core-mantle boundary. The low seismic velocities in the top of the outer core could likely imply the formation of a thermal and/or compositional stratification.