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.
This simulation shows how an earthquake's energy radiates through a cross-section of the Earth as two distinct body waves: fast compressional P-waves and slower shearing S-waves. Wavefronts expand from the epicentre across nested layers — inner core, liquid outer core, mantle and crust — while a ring of six seismograph stations records each arrival. Because S-waves cannot travel through liquid, the model reproduces the S-wave shadow zone that first revealed Earth's fluid outer core.
An animated 2D cross-section of Earth's interior with five layers. P-wavefronts (blue) and S-wavefronts (red) expand from the epicentre as growing rings. The number of rings scales with the chosen magnitude, and S-wave arrivals are suppressed within the shadow zone (modelled from roughly 104° to 140° from the epicentre on each side) to mimic refraction at the core-mantle boundary.
Press "Trigger Earthquake" or click anywhere on the globe's surface to set a new epicentre; you can also drag the epicentre marker. The Magnitude slider (M5.0-M9.0) sets the wave strength, and the Speed slider (0.5x-3.0x) controls playback. Toggle "Show P-waves", "Show S-waves" and "Shadow Zone" to isolate each feature. The info bar tracks elapsed time and how many of the six stations have been hit.
Richard Dixon Oldham identified the seismic shadow zone in 1906, deducing that Earth must have a distinct core. In 1936 Inge Lehmann analysed faint waves arriving inside that zone and concluded the core has a solid inner part surrounded by liquid.
P-waves (primary waves) are compressional: the ground moves back and forth along the direction the wave travels, and they pass through solids and liquids alike. S-waves (secondary waves) are shear waves that move the ground at right angles to their travel direction, and they can only move through solids. P-waves are faster, so they always reach a station first, which is why they are shown in blue and arrive ahead of the red S-waves here.
S-waves cannot pass through liquid, and Earth's outer core is molten. When the path to a distant station would cross the liquid outer core, no direct S-wave can get through, creating the S-wave shadow zone. In this simulation that zone spans roughly 104° to 140° of arc from the epicentre on each side, and stations within it stay marked as not hit by an S-wave.
The Magnitude slider runs from M5.0 to M9.0 and sets how many wavefront rings are released and how large the seismograph wiggles are, so a bigger quake produces stronger, more numerous waves. The Speed slider, from 0.5x to 3.0x, simply scales how fast the animation plays back; it does not change the physics, only how quickly you watch the waves expand and reach the stations.
It is a teaching model rather than a precise geophysical solver. The layer structure, the fact that S-waves are blocked by the liquid outer core, and the existence of a shadow zone are all real. However, the wavefronts are drawn as simplified expanding circles and the shadow-zone angles and wave speeds are approximations chosen for clarity, so it conveys the concepts correctly without computing exact ray paths.
By comparing when and where P-waves and S-waves arrive at stations around the globe, seismologists infer the size, depth and physical state of each internal layer. The pattern of missing direct waves in the shadow zone revealed the liquid outer core, and subtle waves that bend into that zone revealed the solid inner core. In effect, every earthquake acts as a natural scan of the planet's interior.