Fine-scale topography of the D″ discontinuity and its correlation to volumetric velocity fluctuations

We constrained the correlation of the topography of the D″ discontinuity to volumetric velocity fluctuations. We used data obtained from Hi-net tiltmeters deployed by the National Research Institute for Earth Science and Disaster Prevention, Japan, and conducted stacking and migration to map the discontinuity beneath the southwestern Pacific. We compared the obtained map with the distribution of ScS–S travel time residuals, an index of volumetric velocity fluctuations in the lowermost mantle. We detected the fine-scale (of the order of a few hundred kilometers) topography of the D″ discontinuity as well as fine-scale volumetric velocity fluctuations. Regions with relatively higher velocities generally have a discontinuity with shallower depths, and those with relatively lower velocities generally have a discontinuity with deeper depths. The scattering plot between the observed depths of the discontinuity and the observed ScS–S residuals appears to exhibit a nonlinear trend. These results may suggest the existence of both thermal and chemical anomalies, such as those seen in thermochemical plumes.