Synthetic seismograms for a synthetic Earth: 
Computing 3-D wavefields in mantle circulation models to test geodynamic hypotheses
directly against seismic observations


Understanding the dynamic behaviour of Earth’s mantle is of fundamental importance as
mantle flow drives plate tectonics and controls the way the Earth loses its heat.
However, the origin of seismic heterogeneity and the nature of flow in the mantle
still remain elusive.  Current interpretations of seismic observations typically
argue for significant chemical heterogeneity being present in the two large low shear
velocity provinces under Africa and the Pacific. Recently, however, it has been
suggested that large lateral temperature variations in the lowermost mantle resulting
from a high core heat flow and associated strong thermal gradient across D$''$ may
provide an alternative explanation.

Recently, we developed a new joint forward modeling approach to test geodynamic
hypotheses directly against seismic data: Seismic heterogeneity is predicted by
converting the temperature field of a high-resolution 3-D mantle circulation model
into seismic velocities using thermodynamic models of mantle mineralogy. 3-D global
wave propagation in the synthetic elastic structures is then simulated using a
spectral element method. Being based on forward modelling only, this approach allows
us to generate synthetic wavefields and seismograms independently of seismic
observations. Another key advantage is that our approach avoids the problems of
limited resolution and non-uniqueness inherent in tomographic inversions while taking
all possible finite-frequency effects into account. 

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
and uncertainties in the mineralogical parameters are taken into account. This is a
strong indication that seismic heterogeneity in the lowermost mantle is dominated by
thermal variations on the length-scales relevant for long-period body waves.

Finally, we study wavefield effects of direct P- and S-waves in the elastic and
isotropic 3-D seismic structures derived from our geodynamic model. More
specifically, we quantify the dispersion of traveltime residuals caused by
diffraction in structures with dynamically constrained length-scales and magnitudes
of the lateral variations in seismic velocities and density. To this end, we have
created a synthetic dataset of finite-frequency traveltime residuals measured in four
different frequency bands. One question that we wish to adress is whether a comparison
of predicted and observed traveltime disperson can be used to further constrain the
relative contributions of thermal and chemical variations to the seismic heterogeneity in
Earth's deep interior.