Analysis of Seismic Data from the USArray Project to Determine Crust and Uppermost Mantle Structure Beneath the United States

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During the last ten years, NSF's USArray Project has deployed thousands of seismometers across the United States to record both local and distant earthquakes. Analyses of these data have provided greatly improved images of the structure of Earth's crust and mantle beneath the United States. This project studies earthquakes and quarry blasts recorded by USArray at distances up to 1600 km away (1000 miles), which are particularly useful for probing crustal thickness and the structure of the uppermost mantle, i.e., immediately below the crust at depths of about 35 to 50 kilometers (about 20 to 30 miles). Maps showing variations in the measured velocities of seismic waves in this region provide clues regarding the history and evolution of mountains, faults, and other geological features, including the Colorado Plateau and Yellowstone volcanism. This project builds on previously NSF funded work in the western United States to focus on more recent USArray data from the eastern United States.In particular, the project uses regional seismic phases such as Pn, Sn, Pg and Lg, to produce new maps of crustal thickness and both isotropic and anisotropic variations in uppermost mantle P and S velocity that can be related to many surface tectonic features. These results generally agree with previous studies in their large-scale patterns but show more detail in many regions. Owing to its relatively sparse station coverage and lack of earthquakes, the eastern United States has been much less studied than the west, so the most recent USArray station deployments provide a valuable opportunity to probe its crust and uppermost mantle structure. This project will provide new crustal thickness maps, estimates of crustal Vp/Vs ratios, maps of uppermost mantle variations in seismic velocity and azimuthal anisotropy. In addition, analyses of Sn propagation efficiency and Pg and Lg times and amplitudes are planned to resolve lower crustal velocities and attenuation. These results will complement shear-wave splitting and surface- and body-wave tomography studies by providing superior resolution for structures at lower crustal and uppermost mantle depths.