Types of Seismic Waves: P, S, and Surface Waves

Drop a pebble in a pond and ripples spread outward. An earthquake does the same inside the Earth — sending out waves that squeeze, shear, and shake.

Uni Year 1Earth science
⏱️ About 18 min
Types of Seismic Waves: P, S, and Surface Waves — illustration
Illustrative image (AI-generated).

Drop a pebble in a pond and ripples spread outward. An earthquake does the same inside the Earth, sending out waves that travel through rock and along its surface. Some waves squeeze and stretch the rock like a slinky; others shear it sideways. One type cannot pass through liquid — and that single fact tells us the outer core is molten.

💡
The big idea: Earthquakes generate three main wave types. <strong>P-waves</strong> are fast compressional waves that travel through both solids and liquids. <strong>S-waves</strong> are slower shear waves that only pass through solids. <strong>Surface waves</strong> arrive last but cause the most shaking and damage. Because S-waves cannot pass through the liquid outer core, they cast a shadow zone on the opposite side of Earth — direct evidence for a molten core.
🎯 By the end, you'll be able to
  • Contrast P-waves, S-waves, and surface waves by speed, particle motion, and the materials they travel through
  • Explain why S-waves do not travel through liquids
  • Use the S-wave shadow zone as evidence for a liquid outer core
  • Calculate approximate travel times given distance and wave speed
📎 Helpful to know first

P-waves — the fast compressional messengers

P-waves (primary waves) are the fastest seismic waves. They travel by compressing and rarefying the rock in the direction of travel — like a coiled spring being pushed and pulled. Because they involve only compression, they can travel through both solids and liquids. In the crust, P-waves move at roughly 5–7 km/s; in the mantle they speed up to about 8–14 km/s.

P-waves arrive first at any seismograph, which is why they are called 'primary.' They are usually felt as a sharp jolt — a sudden up-down or back-forth motion.

S-waves — the slower shear waves

S-waves (secondary waves) arrive after P-waves. They travel by moving particles perpendicular to the direction of wave travel — a shearing motion, like whipping a rope up and down. Because shear requires a material with some resistance to shear deformation (a property solids have but liquids lack), S-waves cannot travel through liquids.

In the crust, S-waves move at roughly 3–4 km/s — about 60 percent of the P-wave speed. Their arrival is often felt as a rolling, side-to-side motion.

⚠️ Misconception: S-waves travel through everything
S-waves are shear waves. Shear requires a material with internal rigidity — something that 'remembers' its shape. Liquids have zero shear strength: if you try to apply a sideways push to water, it just flows. That means S-waves cannot pass through liquids. When seismologists discovered that S-waves from earthquakes on one side of Earth do not arrive on the opposite side, they had direct proof that a liquid layer — the outer core — blocks them.

Surface waves — the slowest and most destructive

When P-waves and S-waves reach Earth's surface, some of their energy is trapped near the surface and converted into surface waves. These travel more slowly than body waves but often cause the most damage because they move the ground in complex ways and their energy is concentrated near the surface.

There are two main types:

  • Love waves move the ground side to side, perpendicular to the direction of travel, like an S-wave trapped at the surface.
  • Rayleigh waves move the ground in a retrograde elliptical motion — the surface particle traces an ellipse opposite to the direction the wave travels (up and backward, then down and forward).

Surface waves arrive last on a seismogram but produce the largest amplitudes and the greatest structural damage.

P-waves, S-waves, and surface waves radiating from an earthquake focus Surface Focus P-waves (fast, compressional) S-waves (slower, shear) Surface waves (slowest, most damage) Seismic wave types from an earthquake focus
Seismic wave radiation from an earthquake focus. P-waves (fast, compressional) arrive first, followed by S-waves (slower, shear), then surface waves (slowest, most damaging).
🔑 Wave speed order: P > S > surface
On any seismogram, the arrivals always follow the same order: P-waves first, S-waves second, surface waves last. The time gap between P and S arrivals increases with distance from the earthquake, which is exactly how seismologists locate the epicenter.
S-wave shadow zone proving the liquid outer core Inner core Outer core (liquid) Focus P-wave bends through core S-wave stops at core S-wave shadow zone The S-wave shadow zone — evidence that the outer core is liquid
The S-wave shadow zone. S-waves cannot pass through the liquid outer core, so they leave a shadow on the far side of Earth — direct evidence that the outer core is liquid.
\[ t = \frac{d}{v} \]
Travel time (t) equals distance (d) divided by wave speed (v). Because P-waves and S-waves have different speeds, their arrival times at a station differ by an amount that depends on the distance to the earthquake.
📝 Worked example: A seismic station is 300 km from an earthquake. If P-waves travel through the crust at 6 km/s, how long do they take to arrive?
  1. Distance d = 300 km.
  2. P-wave speed v = 6 km/s.
  3. Time t = d ÷ v = 300 ÷ 6 = 50 s.
✓ 50 seconds.
✏️ Practice: An earthquake occurs 360 km from a station. How long do P-waves take to arrive if they travel at 6 km/s? (Answer in seconds.)
s
Solution
  1. t = d ÷ v = 360 km ÷ 6 km/s = 60 s.
✏️ Practice: An earthquake is 210 km from a station. P-waves travel at 6 km/s and S-waves travel at 3.5 km/s. What is the P-S arrival time difference? (Answer in seconds.)
s
Solution
  1. tP = 210 ÷ 6 = 35 s.
  2. tS = 210 ÷ 3.5 = 60 s.
  3. Δt = 60 − 35 = 25 s.
🎮 Seismic-Wave Propagator LIVE
Predict first: Watch P-waves, S-waves, and surface waves radiate from an earthquake focus. Which wave type does not pass through the liquid outer core?

Interactive Seismic-Wave Propagator rendering P-waves, S-waves, and surface waves from an earthquake focus, showing the S-wave shadow zone caused by the liquid outer core.

Interactive wave propagator showing P-waves bending through Earth's core, S-waves blocked at the core-mantle boundary, and the resulting shadow zone.

Check your understanding

1. Which seismic wave type can travel through both solids and liquids?
P-waves are compressional waves and can travel through both solids and liquids. S-waves are shear waves and require a material with shear strength, so they cannot pass through liquids.
2. Why is there an S-wave shadow zone on the opposite side of Earth from an earthquake?
S-waves cannot pass through the liquid outer core because shear waves require a medium that resists shear deformation. Liquids have zero shear modulus, so S-waves cannot propagate through them, creating a shadow zone.
3. On a seismogram, which waves usually cause the most damage?
Surface waves travel more slowly than body waves but concentrate their energy near the surface and produce the largest ground motions, causing the greatest structural damage.
🔎 Related lessons
Previous: Earthquakes & Elastic Rebound Next: Magnitude, Intensity & Locating the Epicenter Earth's Layers: Crust, Mantle, Core
✅ Key takeaways
  • P-waves are fast compressional waves that travel through solids and liquids.
  • S-waves are slower shear waves that only travel through solids, not liquids.
  • Surface waves arrive last but cause the most damage because their energy is concentrated near the surface.
  • The S-wave shadow zone — where no S-waves arrive on the far side of Earth — proves the outer core is liquid.
  • Travel time = distance ÷ speed. The P-S time gap increases with distance from the earthquake.
➡️ Seismic waves carry the earthquake's energy to distant stations. By measuring the arrival times of P- and S-waves, seismologists can work out how far away the earthquake was. With three or more stations, they can pinpoint the exact epicenter — and distinguish the earthquake's magnitude from the local shaking intensity. That is the next lesson.
Want to test yourself on this? Try the Science practice tests →
🎓 Go deeper: university courses & trusted references

Handpicked external material for this module — for when you want the full university treatment of structural geology & earthquakes.

External sites are listed for reference only. This course is independent and has no affiliation with, or endorsement from, the institutions named.