Seismic Event Coda-Correlation Imaging of the Earth's Interior

Abstract

Seismic coda waves are the late part of the seismic energy generated by earthquakes. Global coda correlograms are constructed by cross-correlating and stacking seismic event late coda records that are noisy and seemingly useless, but they exhibit many prominent features sensitive to the Earth's internal structure. Thus, the coda correlation rises as a new paradigm in global observational seismology. As a new category of observations, the correlation features, if interpreted correctly, can provide new information about the Earth's interior. How to accurately utilise seismic event coda correlations, for instance, in "global coda correlation tomography," has been controversial and unresolved. Some attempts treat coda correlations as reconstructed seismic waves, which is on a par with methods developed in ambient-noise correlations, for they share similar data processing and computation routines. However, that introduces erroneous interpretation because theoretical analyses have demonstrated fundamental differences in the formation mechanisms of coda correlations and ambient-noise correlations. Therefore, we need a solution, a correct approach, to allow us to use a massive amount of coda correlation observables to increase constraints on the Earth's interior. This thesis consists of theoretical analysis, method developments, and applications for utilising seismic event coda correlations to image the Earth's interior. We first conduct comprehensive analyses to 'dissect' coda correlations for their formation mechanism quantitatively. The analyses reveal the mathematical relationship between coda correlations and the Earth's internal structure. Based on that, we build a novel framework toward global coda-correlation tomography. We verify the new framework in experiments and compare it with the method based on the assumption of seismic wave reconstructions. We illustrate significant inaccuracy in tomographic images can arise if coda correlations are treated as reconstructed seismic waves. Then, in an application, we provide a new class of observations for inner-core shear-wave anisotropy utilizing coda correlations in the new framework. We find that inner-core shear waves travel faster by at least ~5 s in directions oblique to the Earth's rotation axis than directions parallel to the equatorial plane (anisotropy of >0.8%). Our inner core shear-wave anisotropy observations place new constraints on the inner core mineral composition. Finally, we extend the principles to cross-correlations between source events and devise a new way to build global inter-source correlations. We demonstrate that a single seismic station is sufficient to construct a global correlogram. The correlogram exhibits prominent features sensitive to the internal planetary structures. We show implementations to constrain the Earth's and Martian cores' sizes and confirm a large Martian core. This provides a new paradigm for imaging planetary interiors on a global scale with currently realizable resources in planetary missions. Show more