In Earthquakes and Seismic Waves you met the two body waves: fast P waves that push and pull, and slower S waves that shake side to side. No one has ever drilled through the crust; the deepest hole stops at about 12 km. Yet we have a detailed map of the inside, down to the center 6,371 km below us. Seismic waves drew it.
Why it works
Two facts do the work. First, a wave bends (refracts) when its speed changes, just as light bends entering water. Rock gets stiffer with depth, so waves speed up and curve back toward the surface. Second, S waves are shear waves: they need the material to spring back when it is twisted. Solids do; liquids don't. So S waves cannot travel through a liquid, while P waves can, only more slowly.
- The S-wave shadow. Beyond about $103^\circ$ around Earth from an earthquake, no direct S waves arrive. Something liquid and huge blocks them: the outer core.
- The P-wave shadow. From about $103^\circ$ to $142^\circ$, no direct P waves arrive either. P waves slow sharply as they enter the outer core (from about 13.7 to 8 km/s), bend away and land beyond $142^\circ$.
- The inner core. In 1936 Inge Lehmann noticed faint P waves inside the shadow zone. They had bounced off a solid inner core, where P waves speed up again.
Key idea
The layers were found by seismic waves, not drilling. S waves stop at a liquid outer core, leaving a shadow beyond $103^\circ$; P waves bend at the core and leave a shadow from about $103^\circ$ to $142^\circ$. The size of the shadows gives the size of the core.
Worked examples
Worked example: Which station hears what?
Stations stand $60^\circ$, $125^\circ$ and $160^\circ$ around Earth from an earthquake. What does each record?
- $60^\circ$ is less than $103^\circ$: waves reach it through the solid mantle, so it records both P and S.
- $125^\circ$ is between $103^\circ$ and $142^\circ$: in the P-wave shadow and beyond the S limit, so no direct P or S.
- $160^\circ$ is beyond $142^\circ$: P waves arrive after crossing the core, but no S waves.
Answer: $60^\circ$: P and S. $125^\circ$: neither. $160^\circ$: P only.
Worked example: How far is 103°?
Earth's radius is 6,371 km. How far along the surface is $103^\circ$ from an earthquake?
- Circumference: $2\pi r = 2\pi(6,371) \approx 40,030\,\text{km}$.
- $103^\circ$ is $\tfrac{103}{360}$ of a full turn: $\tfrac{103}{360} \times 40,030 \approx 11,450\,\text{km}$.
- Every large earthquake throws a shadow more than 11,000 km away, and stations worldwide see it each time.
Answer: About $11,450\,\text{km}$.
Common mistake
Many people think the layers were found by drilling, or that lava comes up from the core. The deepest borehole, in Russia, reached about 12.3 km: just $0.2\%$ of the way to the center. Lava comes from partly melted rock in the upper mantle and crust. Check yourself: on a globe 20 cm across, the deepest hole would be about 0.4 mm deep.