EVIDENCE FOR PROPORTIONALITY OF P AND S HETEROGENEITY IN THE LOWER MANTLE

To date, most shear-velocity heterogeneity models in the lower mantle have been derived using long-period data. Comparatively little use has been made of the vast International Seismological Centre (ISC) data b ase of shear-wave arrival times. The aim of this study is to use the I SC P and S arrival times to construct global models of P and S heterog eneity in the lower mantle, and then to compare them in order to inves tigate whether, within the limitations of the data distributions, they might be proportional. The advantages of constructing both compressio nal- and shear-wave models from the one data set is that they share si milar resolution properties, and hence are most reliable in the same a reas. We use data from over 21000 events to derive a data set of P and S summary rays whose residuals we invert jointly along with hypocentr al parameters of over 600 summary events. Particular attention is paid to data weighting so that outliers are not given undue influence. Fur thermore, summary rays with high internal variances are downweighted. In order to diminish the effect of model parametrization on our conclu sion, we derive three sets of P and S models expanded in terms of Lege ndre polynomials for their depth variation, and spherical harmonics fo r their lateral dependence. Comprehensive resolution and error analysi s is performed. Correlation coefficients between our P and S models ar e highly significant, averaging approximately 0.7 for our lowest param etrization (245 model coefficients), and 0.5 for our more highly param etrized models. Visual comparisons show strong similarities in areas w here resolution is high and error is small. We also conduct an experim ent in which we derive compressional- and shear-wave heterogeneity mod els from data sets which contain P and S arrivals from the same seismo grams. These data sets sample the mantle almost identically. The resul tant models compare well but correlation drops towards the core-mantle boundary, indicating that there are genuine physical differences in t he lowermost mantle. Our models indicate that the ratio of relative S to P heterogeneities is close to 2. This value is based on both our co mplete and restricted data set models and hence is not highly dependen t on data weighting.