Internal Structure of the Mantle Transition Zone Beneath the Contiguous U.S.: Insights From the 520-km Discontinuity Revealed by Ambient Noise Correlations

We investigate the internal structure of the Mantle Transition Zone (MTZ) beneath the contiguous U.S. by mapping the depth of the 520-km discontinuity (d520) and the relative amplitude of seismic phases associated with the 520-, 410-, and 660-km discontinuities. Specifically, we analyze short-period reflected waves between 3 and 10 s from the MTZ discontinuities extracted from seismic noise correlations. Our results show lateral d520 depth variations, deeper in the western U.S., likely indicating a warmer upper MTZ. Analysis of relative phase amplitude between the MTZ interfaces highlights a strong d520 reflection phase across the central U.S., supporting previous studies that report high seismic velocity contrasts and elevated olivine content. In contrast, the eastern U.S. shows a weaker d520 reflection phase, which may be attributed to a gradual seismic velocity transition and lower water content. The MTZ composition also likely varies across the U.S., with potential basalt accumulation in the southwest due to past subduction events.