CCREM: New Reference Earth Model From the Global Coda-Correlation Wavefield

The existing reference Earth models have provided an excellent one-dimensional representation of Earth's properties as a function of its radius and explained many seismic observations in a broad frequency band. However, some discrepancies still exist among these models near the first-order discontinuities (e.g., the core-mantle and the inner-core boundaries) due to different data sets and approaches. As a new paradigm in global seismology, the analysis of coda-correlation wavefield is fundamentally different from interpreting direct observations of seismic phases or free oscillations of the Earth. The correlation features exist in global correlograms due to the similarity of body waves reverberating through the Earth's interior. As such, there is a great potential to utilize the information stored in the coda-correlation wavefield in constraining the Earth's internal structure. Here, we deploy the global earthquake-coda correlation wavefield as an independent data source in the 15–50 s period interval to increase the Earth's radial structure constraints. We assemble a data set of multiple pronounced correlation features and fit both their travel times and waveforms by computing synthetic correlograms through a series of candidate models. Misfit measurements for correlation features are then computed to search for the best-fitting model. The model that provides an optimal representation of the correlation features in the coda-correlation wavefield is CCREM. It displays differences in radial seismic velocities, especially near the first-order discontinuities, relative to previously proposed Earth-reference models. This is the first application of the earthquake-coda correlation wavefield in constraining the whole Earth's radial velocity structure.