Global high-resolution phase velocity distributions of overtone and fundamental-mode surface waves determined by mode branch stripping

Abstract

SUMMARY We apply the mode branch stripping (MBS) technique of van Heijst & Woodhouse (1997) to approximately 110 000 three-component seismograms.We select high-quality data with the reliability estimate of van Heijst & Woodhouse (1997). We assess the in£uence of diierent selection criteria and remove outliers using smooth degree-12 phase velocity maps. We present ¢nal results in terms of Rayleigh- and Love-wave fundamental-mode and overtone global high-resolution (l~40) phase velocity maps. We determine the optimum damping of the high-resolution maps with a method based on cross-validation. Our fundamental-mode phase velocity maps are generally in good agreement with previous studies, especially with Ekstro« m et al. (1997). They do, however, contain more short-wavelength structure than previous studies as we apply relatively little damping. The Rayleigh wave overtone phase velocity measurements made with MBS are of high quality in broad frequency ranges. The measurements are generally well explained by phase velocity maps, and variance reductions for some modes, after rejecting outliers, are as high as 85 per cent. We compare our global phase velocity distributions to the previous results of Stutzmann & Montagner (1994) and to phase velocity maps predicted by 3-D tomographic mantle models. Agreement of the model predictions with our Rayleigh wave phase velocity maps, in terms of both amplitude and observed structures, is good. For Love waves, the qualityof the measurements made with MBS is not as high as for Rayleigh waves.The variance reductions achieved are lower and the agreement between model predictions and our phase velocity maps is less, especially in the frequency ranges where interference between diierent mode branches is strong. Finally, as an additional check on the quality of our overtone measurements, we present a comparison of the fundamental-mode Rayleigh phase velocity distribution at 40 s and our fourth Rayleigh wave overtone phase velocity distribution at 62 s. These two modes are similarly sensitive to velocity anomalies in the top of the upper mantle. We show that the two phase velocity maps are in very close agreement.