Anisotropic structure of Earth's inner core from noise correlations

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The Earth's solid inner core is formed from the crystallization of the liquid outer core as the Earth cools. The inner core possesses axisymmetric anisotropy with its fast axis aligned nearly North-South due to the preferred alignment of the inner core iron crystals in this direction. Previous studies of the inner core structure have shown great complexities. Thus, understanding the origin of this prominent anisotropy is one of the fundamental questions of earth science today; and imaging the complex three-dimensional anisotropic structure of the inner core is crucial in this endeavor. Unfortunately, seismic imaging of the deep Earth has been limited by the distribution of earthquake and station locations. This project seeks to explore a new technique that extracts a vastly different set of observations for the deep Earth based on correlations of seismic noise (from ambient environment and from earthquake coda). We expect that this improved inner core seismic model will benefit the mineralogical and the geodynamical communities, as it provides constraints on their models. The project has implications for understanding the origin of the anisotropy, composition, dynamics, and evolution of Earth?s core and the inner core in particular. The retrieval of the deep penetrating waves overcomes some of disadvantages of traditional earthquake-based methods, which will have a fundamental impact on revealing the structure of the Earth?s deep interior.This project builds on recent successes in extracting body waves from correlations of either ambient noise or earthquake coda. During the last decade, ambient noise correlation technique has been widely used to study the structure of our planet, in particular surface wave tomography of lithosphere structure. More recent studies have shown feasibility to extract body waves from correlations of ambient noise or earthquake coda. We have recently succeeded in retrieving clear triplicated PKP phases and inner-core arrivals PKIKP2 and PKIIKP2 simultaneously using stacks of empirical Green?s functions from autocorrelations of single station or cross-correlations between data recorded at different stations. These observations allow us to use differential travel times between different phases to eliminate or reduce the influence of mantle heterogeneity and source errors. The new PKIKP2 and PKIIKP2 data suggest a seismic anisotropy in the inner-inner core of the Earth that has a fast axis aligned near the equator and a different form of anisotropy from the North-South aligned anisotropy observed in the outer inner core. In this project, we seek to explore these new types of data to constrain the 3D anisotropic structure of the inner core. The differential PKP travel times between different branches from noise cross-correlations provide vastly different data coverage from existing earthquake data. The differential PKIIKP2- PKIKP2 travel times provide brand new samples of the deepest part of the inner core. Specifically, we will systematically retrieve PKIKP2 and PKIIKP2 simultaneously applying autocorrelations and cross-correlations to all available arrays around the world and systematically retrieve triplicated PKP branches by applying cross-correlations to pairs of station arrays at distances greater than 145 degrees. We will then combine the new noise-based data with earthquake data to model and invert for 3D inner-core anisotropic structure.