Adaptively parameterized surface wave tomography: Methodology and a global model of the upper mantle

Observations of seismic surface waves are a very powerful tool to constrain the lateral structure of the Earth's upper mantle, including its anisotropy, because they sample this region with an almost constant sensitivity along the raypath. Like all global seismic databases, the set of surface-wave data available to us has a geographically inhomogeneous coverage, which leads to difficulties, particularly in the appropriate choice of model parameterization. On a global scale most tomography models today are still parameterized uniformly. No consideration is given to the inhomogeneous data coverage and resulting inhomogeneous model resolution due to under- or overparameterization of many areas. If the local resolving power of seismic data is not taken into account when parameterizing the model, features will be smeared in the final model, with subsequent misinterpretation. Parameterization density has to change locally, for models to be robustly constrained without losing any of the accurate information available in the best sampled regions. We have implemented a new algorithm for upper-mantle surface-wave tomography, based on adaptive-voxel parameterization. High resolution is achieved in regions with dense data coverage, while lower resolution is kept in regions where data coverage is poorer. This way, parameterization is everywhere tuned to optimal resolution, minmizing both the computational costs, and the nonuniqueness of the solution. We illustrate our method, including appropriate regularization operators, and numerical shortcuts to keep computational costs at a minimum. The latter could be potentially enormous since the spacing of our global grid is locally as small as ∼50 km. We apply our method to the derivation of a global model, with resolution particularly enhanced in the european lithosphere and upper mantle. Our results are in agreement with large-scale features which have already been observed in earlier studies, inlcuding e.g. the Trans-European Suture Zone, the Panonian Basin, opening of the Aegean and Western Mediterranean, possible small-scale upwellings under Iberia and Massif Central, subduction under the Aegean arc. The very deep cratonic root underneath southern Finland is a particularly robust feature of our maps.