Robust modeling of both strength and orientation of anisotropic structure is a tantalizing tool for constraining the dynamic processes in the lithospheric and sub-lithospheric mantle beneath North America, directly meeting the Earthscope goals of understanding the active deformation of North America and tying it to deep Earth structure and dynamics. The long term goal of this research is to develop more detailed regional and global models of seismic anisotropy that can both illuminate the dynamic processes of the upper mantle, as well as allowing for direct comparison with geodynamically predicted models. The objective of this EAGER application, which is the next step towards this long-term goal, is to develop a new approach to image dynamic processes in the lithospheric and sub-lithospheric mantle beneath North America with a novel technique combining both body wave and surface wave observables that allows for finite-frequency vectorial tomography. The international collaboration proposed here is uniquely suited to bring these approaches the PI and the research group of S´ebastien Chevrot have been working on together and take advantage of the incredible dataset offered by the USArray transportable array to move towards obtaining an unprecedented model of anisotropic structure beneath North America. The project will develop these models by pursuing the following specific aims. 1) Perform synthetic testing of joint inversions using this new approach. The inversions will allow us to gain unprecedented information about both strength and orientation of anisotropy. 2) Assemble and process USArray shear splitting and surface wave data and merge and improve existing software between the two research groups. Dense data coverage of both splitting intensity and multi-taper fundamental mode phase delay will be possible, and since both approaches have been developed from a consistent elastic parameterization, merging the software and performing joint inversions of the data shows great potential for greatly improving the capabilities of anisotropic modeling.
" was a 2 year project with the goal of testing a new approach for the joint modeling of two different kinds of seismic waves, surface waves which propagate along the surface of the Earth, and SKS waves, which travel from an earthquake through the Earth's core up to stations on the surface. The project also aimed to gather a new dataset of SKS splitting intensity data for the extensive set of seismic stations of the Earthscope USArray Transportable Array in the western and central United States. The planned modeling approach was based on theoretical work by the Principal Investigator, Mark Panning, for the surface waves and previous work by Sebastien Chevrot, a French National Center for Scientific Research (CNRS) researcher based in the Observatoire Midi-Pyrenees in Toulouse, France, for SKS data. The goal of this modeling is to image the directional dependence of seismic shear wave velocity beneath North America, which can be related to the dynamic ongoing deformation in the mantle and tectonic history of North America. While the gathering of new surface wave data, and calculation and modeling of synthetic surface waves in the ongoing efforts to improve the proposed modeling approach were a major focus of the project, the most important scientific product of the project is the extensive dataset of SKS splitting at over 1400 USArray stations (see included figure). These SKS splitting intensity measurements, which are sensitive to directional dependence of shear wave velocity beneath North America, represent an unprecedented dataset of dense, spatially coherent splitting measurements. While other databases of splitting derived from more classical techniques are available, this is the first derived from splitting intensity measurements, which allow for the use of data from more events and produce more stable results, as demonstrated by the good spatial coherence of independent measurements across tectonic regions. Even before modeling efforts, the dataset has clear implications for the origin of such directional dependence, with large regions showing good agreement with modeled absolute plate motion (APM), suggesting ongoing shear deformation in the deforming asthenosphere beneath the rigid North American lithosphere. Deviations from this orientation, however, are also clear, such as along the Rocky Mountain front, where flow is likely diverted due to large gradients in lithospheric thickness. Amplitude of the signal in the central United States appears to be strongly related to whether lithospheric texture, as predicted from magnetic measurements, is aligned with APM, with large signals in the Superior province beneath Minnesota and Wisconsin, where lithospheric texture aligns with APM, in contrast with weaker signals immediately to the west, where the textures do not align with APM. In terms of broader impacts, this research played an important role in the education of two graduate student scientists, including fostering new international collaboration with the French research group of Sebastien Chevrot. The dataset will be made available to other researchers, both within the field of seismology, as well as other interested researchers in the fields such as geodynamics and tectonics, after initial publication.
