Seismically fast geodynamic mantle models

We show that biased sampling of Earth structure by body-waves provides an additional explanation for the fact that short period body-wave seismic velocity models are faster than long-period free-oscillation models (apparent dispersion). We do this by tracing a set of body-waves used in global tomography studies through synthetic seismic models derived from mantle circulation models. The histograms of the arrival time residuals have a negative mean for all the models investigated. We interpret that this results from a predominance of rays sampling the fast structures of subduction zones due to the concentration of sources there. The interpretation successfully passes two tests; the first showed that the signal is tectonically controlled, while the second involved breaking the correlation between ray paths and structures when no bias is found. The negative mean implies that Earth as sampled by body-waves is fast compared to an average reference Earth (e.g. as measured from free-oscillations). This effect (and others) will need to be quantified before attenuation can be extracted from the apparent dispersion.