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Figure 3. Depth slices of shear wave velocity perturbations of models PLH15, PM15, SLK15, SR16, and DNA13 (rows; top
to bottom) at 5-km, 15-km, 5-km above the Moho, and 5-km below the Moho (columns; left to right), with LU (white) high-
lighting the location of the Llano Uplift.

similar 1D shear velocity models for the depths of 75 km, 150 km, 400 km, and All four average regional models
western U.S. (WUS), 28°–50° N by 102°– 415 km are presented in the GSA Data (Fig. 2B) show the Moho at deeper depths
130° W; the central U.S. (CUS), 36°–50° N Repository (see footnote 1). than the reference 1D models. CUS has
by 90°–102° W; and the eastern U.S. the highest velocities in the crust and
(EUS), 36°–50° N by 70°–90° W (Fig. 2B). RESULTS AND upper mantle, which is consistent with the
In addition to the original data set, models INTERPRETATIONS fact that it largely represents the cratonic
by Schmandt and Humphreys (2010), James core of Laurentia. Due to the presence of
et al. (2011), Obrebski et al. (2011), and Velocity Models for the Southern U.S. thick sedimentary basins in the southern
Chai et al. (2015) were used for the western and Comparison to Other Regions U.S., SUSavg has the lowest velocities in
U.S.; results from Chen et al. (2016) were Root-mean-square velocities (V rms) as a the uppermost crust. In the upper mantle,
used for the central U.S.; and the model by function of depth for the 3D models intro- however, the western U.S. (WUS) is the
Savage et al. (2016) was used for the east- duced above are shown in Figure 2A. The slowest of all models. Despite the lack
ern U.S. Figure 3 displays model perturba- models fall into two distinct groupings: of recent tectonism in the southern U.S.
tions with respect to SUSavg at four depths: (1) PLH15, SLK15, SR16, and SL14 and margin, the SUSavg model is more simi-
5 km and 15 km plus 5 km above and 5 km (2) PM15, NA07, YFCR14, and DNA13, lar to the WUS model for the tectonically
below the Moho, with cooler colors (blue with as much as ~0.75 km/s difference active western U.S. than any of the
and green) representing faster regions and between the two groups at upper mantle other models. Interestingly, Gulf Coast
warmer colors (red and yellow) represent- depths. The average of all models is shown Q 0-values (Q at 1 Hz) are also consider-
ing slower regions. Authors of each model in red (SUSavg). Comparable regional aver- ably lower than mid-continent values
indicate their best guess of the Moho depth ages for the eastern, western, and central but similar to those in the western U.S.,
throughout their model, so the last two U.S. are shown in Figure 2B, along with the with boundaries that correspond to the
panels represent different absolute depths SUSavg, and the 1D reference models, Oklahoma-Alabama Transform and
for each model. Perturbations of models at ak135, and PREM. Ouachita thrust (Cramer, 2017). The

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