An earthquake sends two kinds of waves through Earth's interior: fast compressional P-waves and slower shearing S-waves. They bend, reflect and convert as they traverse different layers — and their patterns on the far side reveal the existence of Earth's liquid outer core.
P-waves (compressional) travel through liquid and solid; S-waves (shear) cannot pass through liquid. The shadow zone — a ring 103°–142° from the epicentre where no direct P-waves arrive — is caused by refraction at the core-mantle boundary. S-waves never emerge beyond 103°.
Click a point on the Earth cross-section to trigger an earthquake. Watch P (red) and S (blue) wavefronts propagate along ray paths computed with Snell's law. Toggle Layers to show the crust/mantle/outer-core/inner-core velocity profile.
Richard Oldham first identified the shadow zone in 1906, inferring Earth has a fluid core. Inge Lehmann discovered the solid inner core in 1936 from a faint wave arrival inside the shadow zone. Both discoveries used seismograph data — the same physics this simulation shows.