Collaborative Research: Shear-wave Splitting and Mantle Dynamics of the North American Plate

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Non-technical DescriptionNumerous geological features and phenomena observed on the surface of the Earth -- such as giant mountain belts, deep ocean basins, earthquakes, and volcanoes -- are the results of dynamic processes operating in the Earth's crust, mantle, and core. Therefore, an improved understanding of these processes are essential for the understanding of the origin and development of the Earth and for mitigation of natural hazards. At the present time, however, only the shallowest part of the Earth can be observed directly using cores from drill holes; consequently, an array of computing-intensive geophysical techniques are applied to image the active planet. Those techniques are similar to those used in the hospital to image the human body.Technical DescriptionShear-wave splitting is a fundamental geophysical observation that uniquely indicates the presence of azimuthal anisotropy along the ray path. Such anisotropy is the result of dynamic processes in the Earth's asthenosphere and lithosphere. However, dynamic interpretation of shear-wave splitting requires homogeneous, high quality measurements, and quantitative forward models. This project will measure shear-wave splitting parameters at all of the USArray Transportable Array and other broadband seismic stations in the eastern U.S. and update measurements throughout the North American plate. The data product includes ~10,000 shear-wave splitting measurements that we are merging with ~16,000 measurements that we produced for the western and central U.S. with NSF prior funding via EarthScope. We are utilizing a set of procedures for reliably measuring and ranking the splitting parameters for producing a uniform database, quantifying shear-wave splitting complexity and likely depth of anisotropy along the way. To complement this reference shear-wave splitting database, we are computing a large number of geodynamic forward models of synthetic shear-wave splitting and anisotropic structure for surface wave studies by computing fabrics from mantle flow and simplified tectonic models. This way, we can test which hypotheses for the origin of anisotropy can be reliably discerned, which tectonic scenarios best reflect observations, and how to guide regional refinement of our understanding of continental and upper mantle structure and dynamics. Using this joint approach, we are addressing a number of significant questions regarding the origin of seismic anisotropy, including lithospheric inheritance and recent asthenospheric flow, and the nature of mantle convection underneath the North American plate. In addition, all our measurements, as well as synthetic waveforms, are being openly shared so as to allow others to refine their resulting reference models; this ensures our data product will have a significant impact on a broad range of Earth science questions.