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

Recent advances in global imaging have lead to tomographic mantle models with regional scale details. To improve these models further, we have extended the usual linearized approach to traveltime tomography to non-linear tomography. Here 'non-linear' means that seismic ray bending due to inferred velocity 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 following 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 relocation of all events in each of these models. The main results are as follows. We observe no overall dramatic change in anomaly patterns, but subtle changes on the global mantle scale lead to a small increase in variance reduction and model amplitudes. These small changes together with very similar resolution estimates for the linear and non-linear inversions do not allow us to investigate formally possible model improvements. However, expected non-linear effects such as the focusing of structures and a baseline shift towards lower velocities indicate an improved solution, which is also more consistent with expected physics than a fully linearized inversion. Apart from that, some very strong changes occur in distinct upper mantle regions such as below Japan, Tibet, South America, Europe and Tonga-Fiji, where 3-D ray bending effects are substantially larger than 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.