FREQUENCY-DEPENDENCE OF THE VISIBILITY AND DEPTHS OF MANTLE SEISMIC DISCONTINUITIES

The visibility and apparent position of a seismic gradient zone depends on the seismic wave frequency at which it is observed, which we illustrate by showing how the visibility and depth of the olivine alpha-->beta phase transformation (410 km discontinuity) change as temperature varies. Temperature variations change bottomside reflection coefficients by factors of 2-4 in the 0.5-1.0 Hz frequency band, which can explain spatial variability in short period P'P-410' observations and the small apparent variability in observed 410 km discontinuity depths. Discontinuity depths inferred from the time lags of these reflections are also frequency dependent, leading to a frequency dependence of estimates of mantle lateral thermal variability based on discontinuity displacements. The frequency dependent seismic Clapeyron slopes for the olivine alpha-->beta phase change we compute are 2.04 MPa K-1 at 0.2 Hz and 2.12 MPa K-1 at 0.05 Hz, and differ significantly from thermodynamic slope for the Mg2SiO4 end-member.