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 &agr;→&bgr; 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'410P' 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 &agr;→&bgr; phase change we compute are 2.04 MPa K-1 at 0.2 Hz and 212 MPa K-1 at 0.05 Hz, and differ significantly for thermodynamic slope for the Mg2SiO4 end-member.