We use a multi-taper spectral estimation procedure to measure the frequency dependence of particle motion polarizations of long- and intermediate-period fundamental mode Rayleigh and Love waves propagating across the thermally mature northwestern Pacific. Frequency-dependent polarization parameters are measured for separated time windows for direct body phases, surface wave Airy arrivals, and early and late surface wave coda. The multi-taper method has optimum spectral leakage properties for short time windows and supplies a framework for computing error estimates for polarization parameters. Our initial results show that surface waves between 10 and 100 mHz are strongly refracted by lateral heterogeneity in the northwestern Pacific. The frequency dependence of this refraction appears to be relatively simple. ¿ American Geophysical Union 1989