Seismic Velocity and Density Jumps Across the 410- and 660-Kilometer Discontinuities.
The average seismic velocity and density jumps across the 410- and 660-kilometer discontinuities in the upper mantle were determined by modeling the observed range dependence in long-period seismic wave arrivals that reflect off of these interfaces. The preliminary reference Earth model (PREM) is within the computed 95 percent confidence ellipse for the 410-km discontinuity but outside the allowed jumps across the 660-kilometer discontinuity. Current pyrolite mantle models appear consistent with the constraints for the 410-kilometer discontinuity but overpredict amplitudes for the 660-kilometer reflections. The density jump across the 660-kilometer discontinuity is between 4 and 6 percent, below the PREM value of 9.3 percent commonly used in mantle convection calculations.
Shearer
PM
Institute of Geophysics and Planetary Physics, Scripps Institution of Oceanography, University of California, San Diego, La Jolla, CA 92093-0225, USA. Lawrence Livermore National Laboratory, Post Office Box 808, L-206, Livermore, CA 94551, USA.
Flanagan
MP