How scientist use the measurement of seismic waves across the earth to determine the make-up of the earth's interior

The two main types of seismic waves are body waves and surface waves. Body waves are broken down into P waves (primary waves) and S waves (secondary waves).

P waves are the fastest and can move through solid rock and liquids. These waves are longitudinal waves.

S waves are slower and can only move through solid rock. These waves are transverse waves.

Scientists used P waves and S waves to determine that part of the core of the earth is liquid. When an earthquake happened, both waves could be detected near the earthquake center, but only P waves (which travel through solids and liquids) could be detected on the other side of the earth. That meant the S waves could not move through the liquid outer portion of the earth's core, while the P waves could.