The 410‐km‐depth discontinuity: A sharpness estimate from near‐critical reflections

The abrupt increase in seismic velocity and density about 410 km below the surface of the Earth occurs at the depth where a phase change in olivine is expected [Akaogi et al., 1989; Katsura and Ito, 1989; Ringwood, 1969]. We have assembled recordings from 22 short‐period seismometers in Washington and British Columbia of 35 nuclear explosions in Nevada. These data reveal relatively noise‐free reflections from the “410” discontinuity. Most of the transition is completed within a depth interval of less than 10 km under the western United States. This result adds to the accumulating seismic evidence of a fairly sharp “410” transition.