Non-linear global P-wave tomography by iterated linearized inversion

Recent advances in global imaging have lead to tomographic mantle models wi th regional scale details. To improve these models further, we have extende d the usual linearized approach to traveltime tomography to non-linear tomo graphy. Here 'non-linear' means that seismic ray bending due to inferred ve locity heterogeneity is taken into account in an iterative method in which inversion steps are alternated with 3-D ray tracing to update ray paths and traveltimes. As a starting point for our non-linear inversion we have used the mantle model of Bijwaard et al.(1998) and ray tracing is performed fol lowing Bijwaard & Spakman (1999a). We have not attempted a full exploration of the non-linear nature of the traveltime inverse problem. This would at the very least require tests with different starting models and the relocat ion of all events in each of these models. The main results are as follows. We observe no overall dramatic change in a nomaly patterns, but subtle changes on the global mantle scale lead to a sm all increase in variance reduction and model amplitudes. These small change s together with very similar resolution estimates for the linear and non-li near inversions do not allow us to investigate formally possible model impr ovements. However, expected non-linear effects such as the focusing of stru ctures and a baseline shift towards lower velocities indicate an improved s olution, which is also more consistent with expected physics than a fully l inearized inversion. Apart from that, some very strong changes occur in dis tinct upper mantle regions such as below Japan, Tibet, South America, Europ e and Tonga-Fiji, where 3-D ray bending effects are substantially larger th an in the deeper mantle since model amplitudes fall off rapidly with depth. In the lower mantle, how-ever, increased focusing effects can be observed that may prove important for detailed interpretations.