How do P & S waves give evidence for a liquid outer core?
The seismic shadows are the effect of seismic waves striking the core-mantle boundary. P and S waves radiate spherically away from an earthquake's focus in all directions and return to the surface by many paths. S waves, however, don't reappear beyond an angular distance of ~103° (as they are stopped by the liquid) and P waves don?t arrive between ~103° and 140° due to refraction at the mantle-core boundary.
Seismic shadow zone:
- Area of the Earth's surface where seismographs cannot detect an earthquake after the waves have passed through the earth.
- P waves are refracted by the liquid outer core and are not detected between 104° and 140°.
- S waves cannot pass through the liquid outer core and are not detected beyond 104°
- This information led scientists in the early 1900's to deduce a liquid outer core