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Two kinds of wave race through the Earth. How do they tell us where a quake was, and what is inside?
Earthquake Waves lab · Last updated: 16 October 2026
You set off an earthquake and watch two kinds of wave race through the Earth. By timing when each reaches distant stations they can find where the quake happened, and by tracking where waves go missing they discover what lies deep inside our planet. No one has dug that deep, yet waves let us see it.
What you'll discover
- Earthquakes send out fast P waves, which push and pull, and slower S waves, which shake from side to side.
- The gap between the two waves' arrival times tells a station how far away the quake was.
- S waves cannot pass through liquid, and the places where they never arrive show that Earth's outer core is molten.
The science in plain words
An earthquake happens when rocks along a fault suddenly slip, releasing stored energy as seismic waves. Two main kinds travel through the body of the Earth. P waves, or primary waves, squeeze and stretch the rock in the direction they travel, like a push along a slinky. They are the fastest and arrive first. S waves, or secondary waves, shake the rock from side to side and travel more slowly.
Because P waves outrun S waves, the gap between their arrivals grows the further a station is from the quake. Measuring that gap gives the distance to the source. One station only knows the distance, so it draws a circle on the map. With three stations, the circles cross at one point, which marks the epicentre. This method is called triangulation.
The waves also act like an X-ray of the planet. S waves cannot travel through liquids, and on the far side of the Earth from a quake there is a large region where no S waves arrive. This shadow zone shows that Earth's outer core is liquid. P waves bend as they pass between layers, and careful study of their paths revealed a solid inner core at the very centre. Waves also speed up and slow down as rock changes, mapping the crust and mantle.
Where it fits
- India, NCERT: Goes beyond the NCERT syllabus for classes 6 to 10; it builds on Class 9 Sound for the nature of waves.
- United States: Grades 6 to 12
- Next Generation Science Standards: MS-ESS3-2, HS-ESS2-3
Common questions
What is the difference between P waves and S waves?
P waves are compression waves that push and pull rock in the direction they travel. They are faster and can pass through solids and liquids. S waves shake rock from side to side, travel more slowly and can only pass through solids.
How do scientists find where an earthquake started?
Each seismic station measures the time between the P wave and S wave arriving, which gives its distance from the quake. Drawing a circle of that distance around three stations on a map, the point where all three circles cross marks the epicentre.
How do we know what is inside the Earth?
Mostly from earthquake waves. They change speed and bend at boundaries between layers. S waves cannot pass through liquid and vanish beyond a certain distance, which shows the outer core is molten. P wave patterns revealed a solid inner core at the centre.
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Try it yourself
The Earthquake Waves lab is one of the interactive 3D science labs inside Taz. Change one thing, predict what will happen, then run it and watch. Labs open after you sign in.