Earth · Signature experience
How can we know what is inside Earth without seeing it?
Earthquakes send P and S waves through the planet. By watching where each wave travels—and where it cannot—scientists can infer the properties of Earth’s interior.
Start the seismic-wave experienceTravels through solids and liquids
P waves cross the outer core, changing speed and direction as material properties change.
Cannot continue through liquid
S waves do not cross the liquid outer core, so direct S waves are absent over a wide region.
Evidence for hidden structure
Missing arrivals and refracted paths provide evidence for internal layers.
Look for this first
Watch what changes during the experience
- 01P and S waves leaving the same earthquake
- 02S waves stopping at the liquid outer core
- 03P waves bending as they cross the core
- 04Surface stations that do not receive a particular direct wave
Why does it happen?
These ideas explain what you see
Earthquakes make waves
Sudden rock movement releases energy that travels as seismic waves.
P waves compress
P waves compress and expand material along their travel direction and can cross solids and liquids.
S waves shear
S waves move material sideways and do not travel through liquids.
Different paths reveal layers
Arrivals, missing arrivals, and bent paths provide clues to Earth’s interior.
Key words
Keep the hard words simple
- P wave
- A compressional wave that can travel through solids and liquids.
- S wave
- A shear wave that cannot travel through liquids.
- Outer core
- Earth’s liquid metallic layer surrounding the inner core.
- Shadow zone
- A surface region where a particular direct seismic wave is not detected.
A seismic shadow zone is not a place without earthquakes; it is a region where a particular direct seismic wave does not arrive.
Try it yourself
Switch P and S waves on separately
Rotate the 3D cutaway and compare P-only, S-only, and both-wave views at the liquid outer core.
Science sources