We investigate the magnitude of shear-wave velocity and density jumps across the 410-km and 660-km discontinuities in the Earth's mantle. We estimate SS reflection and PS conversion coefficients from ScS reverberations and receiver-side P-S converted phases, respectively, using broadband seismograms recorded at stations in western Japan. Fractional changes of model parameters that satisfy our observations in the northwest Pacific are (ΔVs,Δ&rgr;)=(8.5%,5.4%) for the 660-km discontinuity, and (4.9%, 1.9%) for the 410-km discontinuity. ¿ 2001 American Geophysical Union