Ambient Noise and Teleseismic Tomography to infer the Physical State and Structure of the Crust and Upper Mantle in the Western United States

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We seek to advance and apply two powerful new complementary methods of surface wave tomography to rapidly accruing data resources in the western US, predominantly from the Transportable Array (TA) component of EarthScope/USArray. The two methods are ambient noise tomography and two-plane wave teleseismic tomography. Ambient noise tomography (ANT) is based on the extraction of surface-wave Green functions by cross-correlating long sequences of ambient or background seismic noise that form part of the random seismic wavefield. Two plane-wave tomography (TPWT) interprets the variation in amplitude and phase of teleseismic surface waves observed across a regional seismic array in terms of phase velocity variations within the foot-print of the array and, importantly, also models corrugations in the incoming wavefield caused by scattering that occurs between the earthquake and array.Both methods measure surface wave dispersion, but in complementary period bands: ANT (6 - 40 sec) and TPWT (25 - 150 sec). Used in combination, the methods are able to produce dispersion curves (Rayleigh and Love wave, group and phase velocity) across the entire TA from about 6 sec to 150 sec period on a 25-50 km geographic grid. This prospect could not have been foreseen by surface wave seismologists as little as 3 years ago. The information content contained in these dispersion curves at the resolution of these methods ($\sim$70 km across the TA) over a region as large as that proposed is unprecedented. The proposed research will interpret the dispersion maps in terms of shear velocity variations within the crust and upper mantle to a depth of 200 km using a Monte-Carlo method, which returns both the 3-D model and uncertainties. The research proposed here will have a profound impact on the understanding of the physical state and structure of the crust and upper mantle beneath the western US.We anticipate significant impacts on studies of regional-scale tectonics as well as the thermal and compositional state of the lithosphere beneath the western US and, by extension, similar continental regions elsewhere in the world. The proposed data processing and integrative inversion methodology will offer a unique educational experience for the students and post-doctoral staff involved. Moreover, the methods applied to the USArray data are applicable elsewhere as other regional arrays are installed; e.g., in Europe and China and also for PASSCAL installations. The research team has a strong track record of making data products, models, and software available to a wide community, and the data sets, dispersion maps, and models that result from this research will be made generally available. The resulting model also will have relevance for seismic hazard assessment, both for ground motion simulation and to improve and extend community models such as the SCEC 3D reference model. Finally, the application of the new high-resolution inversion methods will help to achieve the vision of the USArray?modeling the Earth beneath the US in unprecedented detail. Movies produced by the research team that track the emergence of dispersion maps as the Transportable Array has been deployed are shown regularly at EarthScope review meetings. The results of the proposed research will also be seen as important components in achieving the US seismological community''s vision for EarthScope.