Seismic Wave Observations and the Discovery of the Outer Core
The process by which Earth’s internal structure—especially the existence of the outer core—was revealed through seismic wave observations is a major discovery in Earth science.
When an earthquake occurs, two main types of seismic waves propagate:
- P-waves (longitudinal waves): These waves travel through compression and expansion and can pass through solids, liquids, and gases.
- S-waves (transverse waves): These waves move by oscillating up and down (or side to side) and can travel only through solids, not through liquids.
However, scientists observing seismic waves on the opposite side of the Earth noticed an unusual phenomenon: S-waves were not detected. Since S-waves cannot travel through liquids, they concluded that a liquid layer must exist somewhere inside the Earth.
Do seismic waves bend as they travel?
Yes, they do. As seismic waves move through the Earth, they gradually curve toward the interior. This occurs because pressure and density increase with depth, causing seismic waves to travel faster deeper inside the Earth than near the surface.
This bending of waves due to changes in wave speed is called refraction, and it follows the same principle as the refraction of light in lenses or in phenomena such as mirages.