Seismic Waves
Click anywhere to trigger an earthquake. Watch P-waves and S-waves propagate through layered Earth, refracting at each boundary according to Snell’s law. Different densities mean different speeds — and curved wavefronts.
What’s happening
Body waves
Earthquakes generate two types of body waves that travel through Earth’s interior. P-waves (primary, compressional) are faster and arrive first — they compress and expand rock in the direction of travel, like sound waves. S-waves (secondary, shear) are slower and move rock perpendicular to the direction of travel. Crucially, S-waves cannot propagate through liquids, so they are blocked by Earth’s liquid outer core — creating a seismic shadow zone.
Snell’s law at layer boundaries
When a seismic wave hits a boundary between layers with different velocities, it refracts — changes direction — according to Snell’s law: sin(θ1)/V1 = sin(θ2)/V2. Waves entering a faster layer bend away from the normal; waves entering a slower layer bend toward it. If the angle exceeds the critical angle, total internal reflection occurs and the wave bounces back.
Earth’s layered structure
Layer Depth (km) Vp (km/s) Vs (km/s) Crust 0 – 35 6.0 3.5 Upper mantle 35 – 660 8.0 4.5 Lower mantle 660 – 2900 11.0 6.0 Outer core 2900 – 5100 8.5 0 (liquid!) Inner core 5100 – 6371 11.2 3.6
The simulation uses a 2D cross-section with proportional layer thicknesses and approximate velocity contrasts. Real Earth has continuous velocity gradients within layers, but the discrete-boundary model captures the essential refraction behavior that creates curved ray paths and shadow zones.
Why wavefronts curve
Even within a uniform layer, wavefronts are circles (or arcs). But at boundaries the velocity jump bends rays according to Snell’s law, so wavefronts that were circular become distorted. In deeper, faster layers, rays curve back toward the surface — this is why seismic stations on the opposite side of the Earth can still detect distant earthquakes despite the curvature.
The shadow zone
Between about 104° and 140° from an earthquake’s epicenter, no direct P-waves arrive at the surface — they are refracted into the outer core and bent away. S-waves are completely absent beyond 104° because the liquid outer core cannot transmit shear waves. These shadow zones were key evidence for Earth’s liquid outer core, discovered by Oldham in 1906 and refined by Gutenberg in 1913.