Seismic Shadow Zone: Basic Introduction

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.


Keypoints:

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

Total Time: 1min 48s
Level: Novice

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