How do seismic waves reveal the structure of Earth's interior?
Overall analysis of the propagation of too-weak, weak, strong and too-strong seismic waves from the epicenters of earthquakes, over a long period, reveals the structure of Earth's interior.
Seismic waves travel faster in mantle. Outer core has an extremely low viscous liquid that is quite distinct from liquid. Seismic waves are reflected here. Yet, too-strong primary waves are propagated into the core. Forever, there is scope for improvement in this research.