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

Abstract

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 inversion scheme. The starting model for the 3-D inversion was the long-wavelength SAW12D model derived from inversion of global waveforms. The traveltime tomography leads to the introduction of smaller-scale structure, and a final model in which small-scale detail exists and can be resolved with a smooth interconnection through long-wavelength structure. The two-point ray tracing was implemented using a 'pseudo-bending' approach for a full spherical 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 accurate treatment of the rays improves the resolution of wave speed gradients and 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 with 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 model; for example, solutions can be produced that are designed to introduce minimum differences from a 1-D reference model. A variance reduction of 48 per cent was achieved in the inversions, with considerable benefit from the inclusion of iterative inversion with 3-D ray tracing and the improved quality 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°to 118°. The new global S models retain the general features of models derived by one-pass linearized 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 tomography using arrival time and waveform data, for example the complex slab morphology beneath the Tonga and Kermadec regions and the sharp boundary between slow and fast uppermost mantle regions beneath western and eastern Europe.