On crustal corrections in surface wave tomography

Mantle models from surface waves rely on good crustal corrections. We investigated how far ray theoretical and finite frequency approximations can predict crustal corrections for fundamental mode surface waves. Using a spectral element method, we calculated synthetic seismograms in transversely isotropic PREM and in the 3-D crustal model Crust2.0 on top of PREM, and measured the corresponding time-shifts as a function of period. We then applied phase corrections to the PREM seismograms using ray theory and finite frequency theory with exact local phase velocity perturbations from Crust2.0 and looked at the residual time-shifts. After crustal corrections, residuals fall within the uncertainty of measured phase velocities for periods longer than 60 and 80 s for Rayleigh and Love waves, respectively. Rayleigh and Love waves are affected in a highly non-linear way by the crustal type. Oceanic crust affects Love waves stronger, while Rayleigh waves change most in continental crust. As a consequence, we find that the imperfect crustal corrections could have a large impact on our inferences of radial anisotropy. If we want to map anisotropy correctly, we should invert simultaneously for mantle and crust. The latter can only be achieved by using perturbation theory from a good 3-D starting model, or implementing full non-linearity from a 1-D starting model.     

Towards a lower mantle reference temperature and composition

We aim to constrain the lower mantle geotherm and average composition from 1D seismic models and experimental mineralogy data, explicitly accounting for possible sources of uncertainty. We employ an isentropic third-order Birch-Murnaghan equation of state, which is in excellent agreement with recent ab initio calculations of density and bulk modulus for Mg-perovskite. Furthermore, ab initio and experimental data are reasonably consistent with each other. Modelling the shear modulus is not as straightforward, but is needed because density and the bulk modulus alone do not sufficiently constrain temperature and composition. To correctly predict ab initio calculations for the shear modulus of Mg-perovskite, we needed to prescribe a cross-derivative at zero pressure, which we determined by trial and errors. Unless this ad hoc cross-derivative is confirmed by further experimental results, there seems to be an inconsistency between ab initio and experimental data. Purely experimental data most likely require a non-adiabatic temperature profile, but it is difficult to infer the number and location(s) of the non-adiabatic increase(s). If ab initio data are used, at least one thermal boundary layer seems reasonable, but its location depends on the modelling of the iron content. A strong chemical density contrast in the mid-mantle (≥2%) is not supported by ab initio data, but is possible with experimental data. Other major sources of uncertainty are the trade-off between thermal and compositional effects, the possible influence of aluminium perovskite, and poorly understood frequency effects.