<p>The elasticity of seifertite is crucial for understanding seismic velocity anomalies in the lowermost mantle. Using density functional theory, we calculated its thermal elastic properties under mantle conditions. Seifertite exhibits the highest seismic velocities among major minerals at the lowermost mantle, with elastic anisotropies comparable to post-perovskite, indicating its important contribution to ultra-high velocity zones and seismic anisotropy at the core-mantle boundary. The transition from CaCl₂-type silica to seifertite at ~2500 km causes a ~3.0% decrease in shear wave velocity but a negligible effect on compressional wave velocity, providing a reasonable explanation for the negative velocity discontinuities above the D” discontinuity. We further evaluated the density and seismic signature of the oceanic crust, finding it exhibits higher wave velocities and velocity gradients than the surrounding mantle. Even using the controversial low-velocity data of davemaoite, the oceanic crust still cannot generate low compressional wave velocity of the large low velocity provinces.</p>