The Seismic Structure, Thermal State, and Anisotropy of the Crust and Uppermost Mantle Beneath the Contiguous US

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Previous investments in surface wave data processing and inversion methodologies together with the growth of the USArray Transportable Array (TA) provide the unprecedented opportunity to obtain an observationally constrained, uniformly processed view of the crust and uppermost mantle across the entire US. By the end of the grant period, the TA will extend across the entire US and the proposed research will produce a synoptic view of the seismic and thermal structure of the North American continent. The primary focus is to construct 3D models of radial and azimuthal anisotropy in the crust and uppermost mantle across the contiguous United States using surface waves extracted from ambient noise and earthquake data as well as receiver functions. Thermal modeling of isotropic velocities via thermodynamically constrained inversions will allow estimates of uppermost mantle temperature, mantle heat flux, and the thickness of the thermal lithosphere across the continent. All inversions leverage recent advances in surface wave tomography methods (e.g., eikonal and Helmholtz tomography) to improve the fidelity of estimated wave speeds. A key component of this work is the estimation of uncertainties using Monte Carlo and related Bayesian statistics from primary measurements to the final seismological models.The proposed research possesses a substantial technical component, involving the application of recently developed methods of seismic data processing and tomography to USArray TA data observed between 2011 and 2015. The scientific motivations, however, are (1) to improve understanding of crustal and uppermost mantle anisotropy, which holds the key to knowledge of the deformation undergone by the continent and the vertical distribution of mechanical coupling in the crust and uppermost mantle, and (2) to illuminate the thermal state of the uppermost mantle. Ambient noise provides unique sensitivity to crustal structure and the ability to separate crustal from mantle anisotropy. The introduction of receiver function data further improves the vertical resolution of the model. Thermodynamically constrained inversions promise new information on the thermal state of the uppermost mantle across the entire contiguous US.The seismological models and the data used to construct them will be archived and shared with the broader geoscience community. A collaboration with the IRIS-DMS has allowed the DMC to archive and distribute ambient noise cross-correlations and continued development of similar products is envisioned. Higher level data products such as dispersion maps, 3-D models, receiver functions, and research codes are delivered to the general public through dynamically updated, internet-accessible databases at CU-Boulder: http://ciei.colorado.edu/ritzwoller_m. The proposal will support two PhD graduate students. Finally, the PI is collaborating with the USGS to use the 3-D models from CU-Boulder to improve location capabilities for small earthquakes across the US. The proposed research will allow these improvements to extend into the central and eastern US where the proposed research will construct a high resolution 3-D model of the crust and uppermost mantle, providing a lasting element to the legacy of the USArray TA.