Mapping Fine-Scale Structure in Earth's Inner Core With a Global Array of Seismic Arrays

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After the Earth formed it quickly differentiated into a dense, fluid, metallic core surrounded by a magma ocean of less dense, molten rock. The magma ocean ultimately solidified into Earth's mantle and crust. These rocky outer layers act as an insulating blanket and have kept most of the core in a fluid state to the present. But at some point, long ago, the core became cold enough to solidify in the very center of Earth, where the pressure is highest. As the core continued to cool, the solid inner core slowly grew larger. Currently, the radius of the inner core is about 1/3 the radius of the core itself. The aim of the proposed work is to map out the subtle variations in the geologic structure of the inner core using scattered seismic energy that has been recorded by a global network of seismic arrays. These patterns will give insight into how the solid inner core is formed and deformed by dynamic processes in the fluid outer core. The proposed work will be conducted by the PI and a graduate student at the University of Utah in collaboration with early career scientists at Los Alamos National Laboratory (LANL). During the course of the project, the graduate student will visit LANL annually and make contacts within the large and diverse LANL geophysics program. It is likely that early career LANL scientists will also visit the University of Utah during the project, strengthening a lab-university collaboration. The proposed work will leverage the tens of millions of dollars that have been invested in developing and maintaining the International Monitoring System (IMS), which is operated in support of the Comprehensive Nuclear Test Ban Treaty. This project will highlight the value of IMS data for basic science and demonstrate a pathway that other academic researchers might use to gain meaningful access to IMS data.The PI proposes a three-year project to map out large-scale regional variations in the fine-scale structure of Earth's inner core using seismic data recorded by a global array of seismic arrays. The PI will apply array processing techniques to earthquake generated waveforms recorded at the small-aperture (~5-20 km) arrays of the International Monitoring System (IMS) to identify and isolate the subtle coda waves that follow pre-critical reflections from the inner core boundary (PKiKP). Pre-critical PKiKP coda waves are uniquely sensitive to the properties of small wavelength (~1-10 km) scatterers in the inner core that contribute to the well-known, but poorly understood, low Q of the inner core. To better understand the origin of long-wavelength variations in inner core Q, the PI will document regional variations in inner core scattering strength and rigorously compare them to existing quasi-hemispherical models of the inner core that are based on the apparent Q measured from waves refracted through the inner core (PKPDF). Forward modeling of pre-critical PKiKP coda envelopes will be conducted using a previously developed ray-based, single-scattering approach that has been validated against multiple-scattering phonon simulations. The proposed work will, for the first time, map out the scattering strength of the inner core over a complete range of longitudes, with dense sampling of both quasi-hemispheres and the longitudinal transition zones. The results will shed light on the fundamental nature of inner core heterogeneity, and constrain the range of geodynamic models that can explain its origin.