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

The visibility and apparent position of a seismic gradient zone depend s on the seismic wave frequency at which it is observed, which we illu strate 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 coefficie nts by factors of 2-4 in the 0.5-1.0 Hz frequency band, which can expl ain spatial variability in short period P'P-410' observations and the small apparent variability in observed 410 km discontinuity depths. Di scontinuity depths inferred from the time lags of these reflections ar e also frequency dependent, leading to a frequency dependence of estim ates of mantle lateral thermal variability based on discontinuity disp lacements. The frequency dependent seismic Clapeyron slopes for the ol ivine 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 thermodynam ic slope for the Mg2SiO4 end-member.