Intellectual Content and Merit This research aims to develop a new method of seismic tomography called "Eikonal tomography" and apply it to ambient noise and earthquake data recorded on EarthScope/USArray to infer the radial and azimuthal anisotropy structure of the crust and uppermost mantle across the western and central US. The method is based on treating the USArray, including the Transportable Array, the Flexible Array, and the ANSS backbone, explicitly as an array in order to track surface waves that propagate across it. The resulting "phase travel time surfaces" place local constraints on the speed and direction of travel of surface waves, directly revealing azimuthal anisotropy with attendant uncertainties. Eikonal tomography represents a significant improvement in surface wave tomography: it is free from ad-hoc regularization, accurately tracks off-great-circle geometrical waves without iteration, provides accurate error estimates, and presents direct local constraints on azimuthal anisotropy. The method is being applied to ambient noise data observed on the USArray from 2004 through 2010 to estimate isotropic and azimuthally anisotropic phase speeds from 6 sec to 40 sec period. These observations are the basis for the proposed radially and azimuthally anisotropic 3D Vs model of the crust and uppermost mantle. The method is being generalized to Love waves and earthquake data (to provide more information about the mantle). The final results encompass the US from the Pacific in the west through the Great Plains in the east.
The goal of the project is to improve understanding of crustal and uppermost mantle anisotropy, which holds a key to knowledge of the deformation history of the crust and mantle. The development of the 3D model of anisotropy would be impossible without the confluence of new methodology (ambient noise tomography, Eikonal tomography) and a unique seismic observational tool (USArray).
Broader Impacts Educational. The data processing and inversion methodology offers a unique educational experience for a graduate student. Intellectual. The ability to resolve crustal from mantle anisotropy provided by the research helps to clarify the interpretation of SKS-splitting. Significant impacts also are anticipated on studies of regional-scale tectonics as well as the deformational state beneath the western and central US and, by extension, similar continental regions elsewhere in the world. Moreover, the methods developed are applicable elsewhere in the world as other regional arrays are installed; e.g., in Europe and China and for PASSCAL experiments. The research team has a strong track record of making data products, models, software, and training available to a wide community. EarthScope: The research is helping to achieve the vision of the USArray -- modeling the Earth beneath the US in unprecedented detail -- and aims to contribute to achieving the US seismological community's vision for EarthScope/USArray.
