Text S2. Inverted Vs model using the Chen et al. (2016) starting model

The output Vs model using the Chen et al. (2016) starting model is presented in Figure S2, as a comparison to the result using the Herrmann (2013) model. Velocities in the lower crust (3.5 km/s to 5.1 km/s) vary over a larger range than those determined using the Herrmann (2013) starting model, and the difference in the upper limit is more significant than the difference in the lower limit. These results are close to the results obtained by Chen et al. (2016) for the crustal velocity values. Velocities in the uppermost mantle (50-150 km depths) vary between 4.29 km/s and 4.96 km/s, which is close to the result obtained using the Herrmann (2013) model. The above comparison shows that in the resolved Vs model, velocities in the lower crust are more sensitive to the starting model than velocities in the mantle.
The distributions of high- and low-velocity anomalies are consistent with the result obtained with the Herrmann (2013) model, although the magnitudes of low-velocity anomalies appear more significant in this model than the results obtained with the Herrmann (2013) model. On the 24 km and 36 km slices, low-velocity anomalies characterize the Ozark uplift (labeled J), the central Illinois Basin (labeled K), and the Rough Creek Graben (labeled L). Since the magnitudes of these low-velocity anomalies are too prominent, they are considered an artifact introduced by the starting model.