Smooth Crustal Models Derived from Surface Wave Dispersion Data for Waveform Tomography based on the Spectral Element Method

As the frequency band typically considered in global S wave tomography of the earth's mantle does not allow the resolution of crustal structure, but the effect on seismic waves of strong heterogeneity in the crust cannot be neglected, a standard approach has long been to perform approximate crustal corrections, based on normal mode perturbation theory. As we move towards 3D numerical modeling, which can represent the effects of the crust very precisely, we are faced with several issues: 1) uncertainties in the crustal model; 2) heavy numerical costs associated with the thin layer meshing required to accurately compute the effects of the crust; 3) the need to connect the input crustal model, and its large Moho depth variations, with the domain of the tomographic inversion. Several approaches have been proposed to replace layered crustal models by smooth models that predict the same short period surface wave dispersion while significantly decreasing the computation time. Here we have chosen a somewhat different approach, in which, rather than smoothing existing crustal models, we directly develop a smooth 3D crustal model from surface wave dispersion data in the period range 10-100s. A constraint on the model is to properly account for the crustal contribution to the long period waveforms used in our SEM-based tomographic approach. We test our methodology on a dataset covering a large part of the Eurasian continent, which is characterized by large lateral variations in crustal structure.