Tomographic Resolution of Plume Anomalies in the Lowermost Mantle

xue, jing ; zhou, ying ; chen, yongshun john;

Mantle plumes as well as ‘superplumes’ have been imaged in the lower most mantle in tomographic studies. To investigate seismic resolution of deep mantle plume anomalies, we use a spectral element method (SEM) to simulate global seismic wave propagation in 3-D wave speed models and measure frequency-dependent P-, S-, Pdiff- and Sdiff-wave travel time anomalies caused by plume structures in the lowermost mantle. We compare SEM time delay measurements with calculations based on ray theory and show that an anti correlation between bulk sound wave speed and S-wave speed could be produced as an artifact. This is caused by different wavefront healing effects between P and S waves in thermal plume models. The differences in wave diffraction between the two types of waves depend on epicentral distance
and wave frequency. We show that bulk-sound speed structure can not be recovered in ray-theoretical tomographic inversions when the lateral extent of the anomaly is smaller than the size of the Fresnel zone in the lowermost mantle. In addition, an anticorrelation between
bulk sound speed and S-wave speed can be produced in ray-theoretical tomography when the size of the anomaly is less than ∼2000 km; and, the artifacts become more pronounced as the lateral extent of the plume decreases. This indicates a chemical origin of ‘superplumes’
in the lowermost mantle may not be necessary to explain observed seismic traveltimes of core–mantle diffracted waves. The same set of Pdiff and Sdiff measurements are inverted using finite-frequency tomography based on Born sensitivity kernels. We show that wave front healing effects can be accounted for in finite-frequency tomography to recover the true velocity model.