Constraints on the Depth of the 400 km Discontinuity Beneath Southern California from Reflections and Conversions

Abstract The olivine-to-wadsleyite transition near 400 km depth (d400) is sensitive to the thermochemical structure of the upper mantle. We estimate the depth of d400 using three seismic phases with different sensitivities to d400 and to the P-wave and S-wave velocities (VP and VS) above d400. Sv400s and P400s are the teleseismic reflection and conversion at d400. Stacks of cross-correlated late coda from 83 large earthquakes recorded by the Southern California Seismic Network (SCSN) contain a unique signal of the topside p400p reflection off the d400. The joint analysis of the p400p lag time and the travel times of Sv400s and P400s helps mitigate the trade-offs and uncertainties in the estimate of the depth of d400. The three travel times are longer than expected in the preliminary reference Earth model due to low wave speeds in the upper mantle beneath the SCSN and/or a d400 deeper than 400 km. A unique combination of ΔVP/VP and ΔVS/VS can be found if d400 is shallower than about 430 km. A depth of 410–420 km requires concomitant reductions in ΔVP/VP and ΔVS/VS that are similar to the velocity reductions found in the tomographic models REVEAL and GLAD-M35.