NSFGEO-NERC: Global ultralow-velocity zone properties from seismic waveform modeling

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This project aims to image the Earth's deep interior using recordings of seismic waveforms generated by earthquakes occurring globally. Constraining the structure, composition and dynamic motions of the deep Earth, from which we have no definitive rock samples, is crucial to understanding both the forces that are actively shaping our Earth from formation to present, and hazards such as surface volcanism that arise from deep within the Earth. This study focuses on technical development of a new modeling approach combining data analysis and prediction of seismic waveforms, which will be applied to imaging features at the core-mantle boundary called ultralow-velocity zones (ULVZs). The existence of these ULVZs is well documented, and several past studies have linked them to important global Earth processes, such as melting, influx of core iron into the mantle, or the leftover remnants from a molten Earth early in its history. Yet what ULVZs physically represent remains in question. This research is aimed at determining what ULVZs are in terms of their composition, location, and relation to past and present processes inside the Earth. This work will provide crucial constraints on understanding how the Earth formed, what the ongoing dynamic motions within the Earth currently are, and how these motions are related to surface hot spot volcanism such as Hawaii and Yellowstone. This project aims to develop a new seismic waveform modeling approach that will allow for a new understanding of the origin of the complex seismic waveforms we routinely record from earthquakes worldwide and how these seismic waves are sensitive to small-scale features within the Earth. The methods and software developed in this project will be shared openly and will be applicable to a broad range of researchers who may wish to apply these techniques to different target areas. In addition, this research benefits a large area of researchers who work on determining Earth structure and processes and how they relate to surface processes. This project establishes a new international collaborative research effort between the United Kingdom and the USA and will support the training of two post-doctoral researcher fellows, one in the UK and one in the USA.The specific project goal is to develop a transformative joint waveform modeling and data analysis approach to characterize global ULVZ structure. The primary tasks are to (1) collect a new global database of seismic waveforms sensitive to ULVZ structure, (2) use recent developments in seismic wavefield modeling that includes full-wave sensitivity kernels and differential wavefield mapping to determine the sensitivity of seismic arrivals to ULVZ structure and to identify additional seismic arrivals that may be utilized to study ULVZ properties, (3) further the development of wavefield modeling approaches using a fast 3-D Born waveform modeling approach in order to predict seismic waveforms for any desired input model, which allows for (4) the determination of global ULVZ structure through a Bayesian probabilistic inversion, and (5) assess the physical origin of ULVZs by testing current mineral physics models of ULVZs. The approach represents an entirely new line of interrogating localized structures in the deep Earth. The ultimate aim of this work is to fundamentally reassess ULVZs from all angles, and reduce uncertainties in their properties, location, and composition. Ultimately this project will produce a global assessment of ULVZ existence as well as to determine their compositional and geographic scope, and how they are related to other dynamic features inside the Earth.