Direct evidence has been obtained for significant refraction of Rayleigh waves of periods 10-100 sec propagating in the thermally mature western Pacific. Measurements of the frequency- dependent polarization states of Rayleigh Airy phases and early and late coda indicate that the refracted waves are characterized by a banded spectral structure, with different signal polarizations in each band. This banded structure may correspond to the similarly banded frequency dependence of the sensitivity of the fundamental mode to heterogeneity in the asthenosphere, lithosphere, crust, and perturbations to the bathymety. If so, measurement of the frequency dependence of such quantitites as back azimuth and the phase difference between vertical and quasi- longitudinal components may provide discriminants for resolving the distribution of heterogeneity, and possible anisotropy, in the crust, lid, and low-velocity zone.
