Improving global shear wave traveltime tomography using three-dimensional ray tracing and iterative inversion

A fully non-linear approach to global tomography using S-wave arrival times has been implemented using 3-D ray tracing with an iterative linearized in version scheme. The starting model for the 3-D inversion was the long-wavel ength SAW12D model derived from inversion of global waveforms. The travelti me tomography leads to the introduction of smaller-scale structure, and a f inal model in which small-scale detail exists and can be resolved with a sm ooth interconnection through long-wavelength structure. The two-point ray t racing was implemented using a 'pseudo-bending' approach for a full spheric al 3-D model of the mantle. Generally, the ray paths in the full 3-D model and 1-D reference model are quite close, but the inclusion of a more accura te treatment of the rays improves the resolution of wave speed gradients an d the positioning of heterogeneity, particularly near strong variations in wave speed, for example in subduction zones. A further advantage of the use of 3-D ray tracing is that it is possible to undertake resolution tests wi th fewer approximations. With the aid of the non-linear inversion, a number of global S models have been constructed using different assumptions about the character of the mod el; for example, solutions can be produced that are designed to introduce m inimum differences from a 1-D reference model. A variance reduction of 48 p er cent was achieved in the inversions, with considerable benefit from the inclusion of iterative inversion with 3-D ray tracing and the improved qual ity of the data set used in this study. Resolution in the lower part of the mantle has been improved by supplementing the S arrival time data with SKS times for the distance range from 84 degrees to 118 degrees, The new globa l S models retain the general features of models derived by one-pass linear ized inversion with 1-D ray tracing, but provide more focused images with a higher perturbation level for the same damping parameters. The new models are able to provide a good definition of features revealed by regional tomo graphy using arrival time and waveform data, for example the complex slab m orphology beneath the Tonga and Kermadec regions and the sharp boundary bet ween slow and fast uppermost mantle regions beneath western and eastern Eur ope.