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3D Seismic Waves and Earth's Interior – P- and S-wave Shadow Zones

GeographyEarth's Structure & LandformsAges 11–12

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A 3D globe cut in half shows the crust, mantle, liquid outer core and solid inner core with their real depths and temperatures. Place an earthquake focus (drag it along the crust and set its depth) and press Earthquake: P and S rays spread through the Earth and bend at each layer boundary, while S waves stop at the liquid outer core. Thirty-six seismometer stations around the cut and more on the globe change colour as waves arrive, and a travel-time graph and seismogram reveal the S-wave shadow zone (from about 104°) and the P-wave shadow zone (about 104°–140°) – the evidence that the outer core is liquid.

Lesson: Structure of the Earth: crust, mantle and core; how seismic waves reveal the Earth's interior

What it shows

Nobody can drill to the core, so we learn about the Earth's interior from earthquake waves. P waves are compressional and travel through solids and liquids; S waves are shear waves and cannot travel through liquids. The speed of both changes with depth, so rays curve and refract at layer boundaries. Stations more than about 104° from an earthquake receive no direct S waves, which shows that the outer core is liquid, and between about 104° and 140° strong P waves are missing because the core bends them away. Weak P waves that pass through the solid inner core led to its discovery in 1936.

How to use

Drag the red dot to move the focus and set Focus depth, then press Earthquake! and watch the rays spread. Tap a station to see its seismogram and the S – P time, or tap the cut face to read depth, temperature and wave speeds. Choose P waves only or S waves only to compare them, turn Show shadow zones on and off, and drag the background to see the shadow zones on the globe.

Parameters you can change

  • Wave type P and S waves, P waves only, S waves only
  • Focus depth 10–700 km
  • Rays on each side 6–40 rays
  • Time speed-up 30–300 ×
  • Show the shadow zones

Questions to explore

  1. Why do stations on the far side of the Earth record no S waves?
  2. When you change the focus depth, do the shadow zones move?
  3. What does the time between the P and S arrivals tell us about the distance to an earthquake?

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