Multimode surface wave tomography for the Australian region using a three-stage approach incorporating finite frequency effects

A three-stage inversion technique for surface wave tomography is applied to the Australian region. The inversion procedure consists of three independent processes. In the first stage, path-specific one-dimensional (1-D) shear velocity profiles are derived from multimode waveform inversion to provide dispersion information. The information from all paths is then combined to produce multimode phase speed maps as a function of frequency. The first version of these phase speed maps is derived from linearized inversion based on the assumption of surface wave propagation along great circle paths. Subsequently, the 2-D phase speed maps are updated by including ray tracing and finite frequency effects through the influence zone around the surface wave paths over which the phase is coherent. Finally, in the third stage the 3-D shear wave speed distribution is reconstructed from the set of updated multimode phase speed maps. This three-stage inversion of surface waves has significant benefits because it is possible to incorporate multimode dispersion, off-great circle propagation, and finite frequency effects for surface waves in a common framework. The final 3-D model, which includes the effects of ray bending and finite frequency, shows improvement in the definition of the model in regions with high gradients in shear velocity, such as near tectonic boundaries, especially in eastern Australia. Despite the natural smoothing imposed by considering the influence zone around the surface wave paths, the final models still require rapid change in shear wave properties in the neighborhood of the edge of the craton.