Seismic Waves: P-waves and S-waves

When the ground becomes a wave

Deep underground, two slabs of rock that have been locked together for centuries suddenly slip. The stored energy bursts out and ripples through the planet as seismic waves — the waves of an earthquake. By studying them, scientists not only measure earthquakes but also see right into the centre of the Earth, a place no drill could ever reach.

Seismic waves come in two main underground types, and they behave very differently. If you have met transverse and longitudinal waves, you already know the key idea behind them.

P-waves: fast and pushy

P-waves (primary waves) are longitudinal — they push and pull the rock back and forth along the direction they travel, squashing it into compressions and stretching it into rarefactions, exactly like a sound wave.

P-waves are the fastest seismic waves, racing through rock at around 6 to 8 kilometres per second. Because they are first to arrive, they earn the "P" for primary. A crucial property: P-waves can travel through solids and liquids alike.

S-waves: slower and shearing

S-waves (secondary waves) are transverse — they shake the rock from side to side, at right angles to their direction of travel, like a wave on a rope.

S-waves are slower, about 3 to 4 km/s, so they arrive after the P-waves and earn the "S" for secondary. Their defining feature: S-waves can travel through solids only. A liquid has no rigidity to "shear" sideways, so it stops an S-wave dead.

Reading the gap: how far away was it?

Both waves leave the earthquake's focus at the same instant, but the P-wave pulls ahead because it is faster. The farther they travel, the more the speedy P-wave outruns the S-wave, so the time gap between their arrivals grows with distance.

A seismograph records the shaking as a wiggly line. Scientists measure the gap between the P-wave jolt and the S-wave jolt, and that gap tells them how far away the quake was. Combine readings from three stations and you can pinpoint the exact location — a method called triangulation.

Worked example: locating an earthquake

At one station the P-wave arrives, and the S-wave follows 40 seconds later. The P-wave travels at 8 km/s and the S-wave at 4 km/s.

In the time t it takes light... no — let the distance be d. The P-wave takes d ÷ 8 seconds; the S-wave takes d ÷ 4 seconds. The difference is 40 s:

d/4 − d/8 = 40 2d/8 − d/8 = 40 d/8 = 40 d = 320 km

The earthquake happened about 320 km away. Repeat this at two more stations and the crossing point reveals the epicentre.

Seeing inside the Earth

Here is the beautiful part. After a big earthquake, stations on the far side of the planet record the P-waves but pick up no S-waves at all over a wide band called the S-wave shadow zone. Since S-waves cannot cross liquid, something liquid must lie between us and the far side. That something is Earth's liquid outer core. The deep core was discovered not by digging but by listening to waves — a triumph of physics.

Try it yourself! 🧪

Model the two wave types with a Slinky on the floor.

1. Stretch a Slinky along a smooth floor with a friend holding the far end still.
2. P-wave model: push your end sharply toward your friend and pull back. A compression pulse runs along the coils — a longitudinal wave.
3. S-wave model: flick your end sideways. A side-to-side wiggle travels down the spring — a transverse wave.
4. Notice the push-pulse moves faster and more easily than the floppy sideways wiggle, just as real P-waves outrun S-waves.

To see how a related wave — ultrasound — is used to image the inside of the human body, visit echoes and ultrasound.