Fig. 3: Changing ultra-low velocity zone (ULVZ) and lowermost mantle anisotropy inferred from the observed temporal changes in SKS and S-Scd-ScS phases at IU.SDV.

a (Left) Two types of changing ULVZ along the core-mantle boundary (CMB) that explain SKS data: I. shrinking ULVZs (top), and II. moving ULVZs (bottom). An approximate SKS ray path is marked by green arrow. ULVZ edges change from red lines at the occurring time of the earlier event (2000) to blue lines at the occurring time of the later event (2009) between doublet AI-0009, with amounts of lateral change labeled. (Right) Superimposed SKS waveforms of doublet AI-0009, with the top pair for the observations (labeled as Obs.) and the two bottom pairs for synthetics of the two types of ULVZ changing models on the left panel (labeled as Syn. I and Syn. II; synthetics of the two models in each type are indistinguishable and are only plotted for the left model of each type). b (Left) Velocity model of a changing D″ that explains S-Scd-ScS data: SV-wave velocities change with respect to (w.r.t) the Preliminary reference Earth model⁴⁴ (PREM) (labeled as \(\delta {V}_{{SV}}\)) from red line at the occurring time of event 2000 to blue line at the occurring time of event 2009 between doublet AI-0009, with amounts of SV-wave velocity change labeled. (Right) Superimposed S-Scd-ScS waveforms of doublet AI-0009 (red for event 2000 and blue for event 2009), with the top panel for radial components (labeled as R comp.) and the bottom panel for transverse components (labeled as T comp.). In each panel, the top pair show the observations between the doublet (labeled as Obs.) and the bottom pair shows synthetics of the changing D″ anisotropy shown on the left panel (labeled as Syn.). The radial synthetics are calculated based on the red (for event 2000) and blue (for event 2009) models on the left panel, respectively, while the transverse synthetics are calculated based on the red model on the left panel for both events.