Seismologists study the propagation of seismic waves, which are elastic waves that travel through the Earth's interior following the release of energy from an earthquake or explosion. Among these waves are P waves (primary waves) and S waves (secondary waves). P waves are compressional waves that can travel through solids, liquids, and gases, while S waves are shear waves that can only travel through solids. S waves cannot travel through a liquid medium. This is because S waves require a solid material to transmit their side-to-side motion. When they encounter a liquid, they are unable to propagate further. The inability of S waves to travel through the Earth's outer core was a key piece of evidence that led to the conclusion that the outer core is in a liquid state. This discovery was significant because it helped to confirm the liquid outer core theory, which is crucial for understanding the Earth's magnetic field and geodynamo. read more >>

An S wave is slower than a P wave and can only move through solid rock, not through any liquid medium. It is this property of S waves that led seismologists to conclude that the Earth's outer core is a liquid.read more >>