Detailed structure within the Mantle Transition Zone beneath the contiguous U.S.: Insight from the 520-km discontinuity revealed by ambient noise correlations

We investigate the internal properties and characteristics of the Mantle Transition Zone (MTZ) beneath the contiguous U.S. by detecting the presence of the 520-km discontinuity (d520) and determining its depth variation. Observations of d520 are performed using short-period reflected waves extracted from noise correlations (3-10 s period) employing a data selection strategy based on quantitative noise phase composition analysis. Detection of d520 is supported by analysis of the relative energy values between MTZ reflection phases. Our results reveal significant lateral variations in d520 depth, with deeper than average depths observed in the Western U.S. and shallower depths in the Eastern U.S. The Central U.S. exhibits transitional behavior. Analysis of relative phase energies indicates a strong d520 reflection phase across the central U.S., consistent with high seismic velocity contrasts and a likely higher olivine content in this region. In contrast, the Eastern U.S. shows a weak d520 reflection phase, potentially due to a gradual transition and/or potentially lower water content. The Western U.S., characterized by depressed d520 and d410 depths, likely reflects a warmer upper MTZ. The MTZ composition likely varies across the U.S., with potential basalt accumulation in the southwest due to past subduction.