3D-reference boundary sensitivities (b) are dependent on the local structure of the 3D reference crustal model (for example, CRUST2.0, (a), left). They can predict measured phase-delay times caused by additional perturbations in crustal thickness ((a), right) when perturbations are small (less than 5 km), as shown in (c). Iterative inversions of global crustal structure can be carried out by using phase-velocity kernels to account for large perturbations in crustal thickness in the starting 3D crustal model, and using 3D-reference boundary Frechet kernel to account for small additional perturbations in crustal thickness relative to the starting 3D crustal model.

Acknoweldgements: This research was financially supported by the US National Science Foundation under Grant EAR- 1013617.

For further reading: Liu, K., and Y. Zhou, Effects of global crustal structure on surface-wave phase delays, Geophysical Journal International, in review.

Keywords: surface_wave, crustal_structure, nonlinearity, finite-frequency_effects