Seismic Event Coda-Correlation: Toward Global Coda-Correlation Tomography

Seismic event coda-correlation is a mathematical manifestation of the seismic wavefield, and it is characterized by many prominent features that are formed due to the similarity between multiple pairs of seismic phases. This new paradigm sets a stage for extracting valuable information about Earth structure. However, earthquake coda-correlation has a fundamentally different physical mechanism from ambient-noise correlation and thus cannot be utilized in the same way as the ambient-noise correlation tomography that has been rigorously studied both in terms of theory and applications. Therefore, we are motivated to devise a new framework for the coda-correlation tomography, in which relevant features in coda-correlation are decomposed and separate constituents are individually related to Earth structure to build sensitivity kernels for tomography. Our theoretical framework is verified via a toy-problem experiment, and we compare the newly proposed method here with the one based on the assumption that an interreceiver response (Green's function) can be obtained. We illustrate that significant differences can arise in the interpretation of results if the Green's function is used instead of the newly proposed framework based on the understanding of the formation of coda-correlation. The proposed framework paves the way for further detailed and application-oriented method improvements and exploitation of the coda-correlation tomography in global and planetary seismology.