Ray-theoretical and finite-frequency seismic traveltime predictions for tomographic filtering of 3D mantle circulation models

Linking geodynamic models to observations from seismology is essential for improving our understanding of the present-day thermodynamic state of the mantle. From the geodynamic perspective, 3D mantle circulation models (MCMs) yield physically relevant predictions of the global distribution of buoyancy forces, while complementing information is available from seismic data and tomography that can reveal the location and morphology of mantle heterogeneity. Investigating this powerful interplay in a fully synthetic framework has great potential. It allows us to make robust interpretations of mantle structure provided that quantitatively meaningful comparisons can be made. This especially relates to the magnitudes of heterogeneity that can not be effectively constrained by the individual modelling approaches.

Following this general concept, there are two possible links: 1) synthetic seismic data can be predicted from the MCM and statistically be compared against observed data. 2) the MCM gets modified by a tomographic operator (informing us about spatially variable seismic resolution and, if applicable, model uncertainty), and subsequently this filtered version gets compared against the corresponding tomographic image from real observations.

Here, we discuss these two strategies together based on observed data for S-wave cross-correlation traveltime residuals that have been applied to global seismic tomography. Taking the same set of source-receiver configurations, synthetic traveltime predictions are computed in a state-of-the-art MCM using ray theory (RT), paraxial finite-frequency kernels (FFK), as well as cross-correlation measurements on synthetic seismograms (SPECFEM). The latter requires computationally demanding 3D-wavefield simulations using SPECFEM3D_GLOBE for an earthquake catalog comprising over 4,200 teleseismic events.

These data sets can be used for tomographic filtering by application of the generalized inverse operator of the actual tomographic model. Filtered MCMs derived from the differently predicted data sets appear largely similar on a global scale with regards to the shape and amplitudes of imaged mantle heterogeneity. This is observed despite the lack of more accurate wave physics in RT or FFK and possible measurement errors for the SPECFEM data that, although being computed in a synthetic case, can not be completely ruled out. Stronger differences between filtered models appear in regions of higher image resolution where model uncertainty by propagated data errors can play a more prominent role.

We discuss the impact of the different filtering strategies by comparing filtered models to the original MCM and synthetic traveltime residuals to the underlying real observations. The results strongly highlight the need for incorporating both resolution and model uncertainty in combined tomographic-geodynamic studies.