A Single 520 km Discontinuity Beneath the Contiguous United States With Pyrolitic Seismic Properties

Olivine polymorphs are considered the most abundant minerals in Earth and vital to governing its dynamics. Seismic discontinuities near 410 and 660 km depth are attributed to phase transitions of olivine polymorphs and have long been in reference Earth models. However, the significance of the 520 km discontinuity (520) and its causative phase transition are debated. To address its prevalence and properties, receiver functions from >2,000 seismographs across the U.S. were inverted using parameterizations with and without the 520. A 520 is required for 84% of the area at 95% confidence. The 520s depths and S-velocity contrasts nearly match predictions from the pyrolite model, as expected for a widespread feature that dominantly reflects the wadsleyite to ringwoodite transition.