Pn and Sn Tomography of the Western United States Using USArray

USArray has greatly improved tomographic resolution for much of the western United States and enables imaging of lithospheric, as well as deeper structures, at a variety of scales. Taking advantage of the vast amount of local and regional phase arrivals routinely picked by the USArray Network Facility, we first perform Pn tomography using the classic time term method. The Pn inversion results show prominent features of crustal thickness, velocity perturbations, and anisotropy, such as the low velocities in the Snake River Plain near the Yellowstone hotspot track and mostly fault-parallel fast axes in central California. Many of these structures are observed throughout the region from studies using different data sets. Pn (and Sn) tomographic velocity perturbation and anisotropy images show results for a very confined depth in the uppermost mantle, complementing surface-wave or other body-wave tomographies that average anomalies over larger depth intervals. There are already more than 70,000 Sn picks in the USArray database. Preliminary analysis of these data, using the same method as for Pn picks, indicates that there is useful signal in the Sn picks, despite somewhat more scatter in their times. Comparisons between the Pn and Sn results show good agreement in large-scale features. To obtain a better understanding of the nature of the Sn picks, we examine the quality and polarization of the Sn waveforms in the USArray database. In addition, we make supplementary picks, both to fill in gaps in data coverage and to more completely parameterize any polarization and/or splitting information. Finally, we will describe experiments with using the Sn data to perform joint inversions with Pn to obtain a more complete picture of uppermost mantle heterogeneity and anisotropy and to potentially resolve crustal Vp/Vs ratios using the Pn and Sn station and event time terms.