S velocity variations beneath North America

We investigate S velocity variation in the upper mantle beneath North American to better understand the effects of data heterogeneity, model parameterization, and regularization. To this end, we analyzed and fit regional S and Rayleigh wave trains generated by earthquakes around North America that occurred between the years 2000 through 2006, including waveforms from the Transportable Array stations of EarthScope's USArray. These new data were combined with constraints used for the 3-D S velocity model NA04 in order to create a new model, NA07. Another model, NA07, was created from a suite of good-fit models to provide a useful guide for model velocities and uncertainties by estimating ranges of probable velocity variations throughout the upper mantle. We find that the southern and eastern edges of the North American craton appear to be defined by Paleozoic orogens rather than Proterozoic ones. On average, the Archean portion of the craton is ∼200 km thick, while the Paleozoic part averages ∼175 km thick with an ∼80 m/s lower S velocity. The horizontal gradients in velocity are over ∼1.0%/100 km at the western margin of the craton, ∼0.5%/100 km in the south, and ∼1%/100 km at the eastern margin.