Synthetic Seismograms for a Synthetic Earth -- Long-period P- and S-wave Traveltime Variations can be Explained by Temperature Alone

The origin of seismic heterogeneity and the nature of flow in the lower mantle still remain elusive. A number of seismological observations point to significant chemical heterogeneity being present in the two large low shear velocity zones mapped by tomography in the lowermost mantle under Africa and the Pacific. Recently, an alternative explanation for the strong reductions in shear wave speed has been proposed: in case of strong core heating, isochemical whole mantle flow with a pyrolite composition is compatible with the magnitude of shear wave velocity perturbations in tomographic models. The assumption of a substantial core contribution to the mantle energy budget is supported by a large number of geophysical studies. The inference of a dominance of thermal variations is based on isochemical high-resolution mantle circulation models, for which temperatures have been converted to seismic velocity models using thermodynamic models of mantle mineralogy. Most important, the limited resolving power of seismic tomography due to uneven data coverage and errors in the observations has been taken into account. This was done by ``tomographically filtering'' the predicted structures using the resolution operator of S20RTS. The hypotheses of strong core heating, however, has so far been tested only against tomographic S-wave velocity models, the geoid and the rate of true polar wander, but not against P-wave velocity models. Unfortunately, the resolution operator can not easily be constructed for the latter, which typically have a large number of free parameters in the inversion. Therefore, we will explore a new approach to test dynamic flow calculations: We present a synthetic seismic dataset for predicted elastic structures of the mantle, which we obtain from large-scale simulations of 3-D seismic wave propagation through the aforementioned mantle circulation model. The key advantage of our approach is that we avoid the problems of limited resolution and non-uniqueness inherent in tomographic inversions while taking all possible finite-frequency effects into account. Capturing the correct physics of wave propagation allows for a consistent test of the assumption of high core heat flow directly against seismic data. We concentrate on the statistics of long-period body wave traveltime data, which show a markedly different behaviour for P- and S-waves: The standard deviation of P-wave delay times stays almost constant with turning depth while that of the S-wave delay times increases strongly throughout the mantle. Surprisingly, our synthetic traveltime variations from an isochemical mantle circulation model with strong core heating reproduce these different trends. Moreover, the related strong lateral temperature variations in the lower mantle are able to explain most of the standard deviation of observed delay times, when contributions from errors in the real data are taken into account. This is a further strong indication that seismic heterogeneity in the lowermost mantle is dominated by thermal variations on the length-scales relevant for long-period body waves.