S velocity variations beneath North America

Abstract

[1] 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.

Isostatic compensation for a portion of the Southern Appalachians: Evidence from a reconnaissance study using wide-angle, three-component seismic soundings

“…Recent studies of S-wave velocity structure beneath North America suggest either virtually no variation in isotropic velocity at a depth of ~70 km across the study area (Nettles and Dziewonski, 2008) or a small (150 m/s) northwestward increase in velocity at a depth of ~90 km from the coastal plain to the Blue Ridge Mountains of Georgia (Bedle and van der Lee, 2009). The latter increase does not extend into the mountains of North Carolina.…”

Heat Flow and Thermal Structure of the Lithosphere

“…For North America, upper mantle shear wave velocities derived from surface wave tomography (Bedle and van der Lee, 2009) are clearly correlated with surface heat flux. The map of vertical shear wave travel delays calculated between 260 and 60 km shows negative travel time anomalies beneath the craton and positive anomalies in active regions (Figure 23), which can be directly compared with the North American heat flow map (Figure 2).…”

“…Shear wave travel time delays between 60 and 300 km based on the seismic tomography model NA07 of Bedle and van derLee (2009). The delay is the difference between the vertical propagation time of shear waves calculated with the NA07 model and the vertical travel time for the reference ak135 velocity profiles.…”

Crust and Lithospheric Structure - Seismological Constraints on the Lithosphere-Asthenosphere Boundary

“…While the incorporation of surface wave overtones (e.g., Figure 5) will typically improve vertical resolution, these inversions will still have difficulty in differentiating between gradients over $50 km and those over smaller Figure 4 Models of global shear attenuation (left) and shear velocity (right) shown at a depth of 100 km illustrate differences between cratonic regions where the lithosphere is characterized by thick layers with high velocity and low attenuation, and regions with thinner lithosphere. In addition, the magnitude of the velocity drop at the LAB exhibits dependence on the starting model used in the tomographic inversion (e.g., Romanowicz, 2009) and varies considerably between different studies (Bedle and van der Lee, 2009;Kustowski et al, 2008;Lekic and Romanowicz, 2011a,b;Nettles and Dziewoń ski, 2008;Ritsema et al, 2011;Yuan et al, 2011). Isotropic velocities from the radially anisotropic shear-velocity model of Kustowski et al (2008) depth ranges.…”