How do seismic waves reveal the boundaries between Earth’s layers?

Seismic waves are the primary tool for mapping Earth’s interior, as their behavior changes significantly depending on the physical and chemical properties of the materials they encounter. When these waves propagate through the planet, they may speed up, slow down, bend (refract), or reflect at boundaries between layers.12

Scientists analyze these changes by measuring the travel times of P waves and S waves from earthquake sources to recording stations. Key indicators of internal boundaries include:23

• Velocity Changes: As waves pass from one layer to another, their velocity often changes abruptly. For example, seismic waves accelerate as they cross the Mohorovičić discontinuity, the boundary between the crust and the mantle. Conversely, waves slow down when entering the weaker, hotter asthenosphere.41
• Refraction and Reflection: Waves refract or reflect when they hit interfaces between layers of different densities or states of matter.23
• Wave Blocking: S waves cannot pass through liquids because they lack shear strength. The fact that S waves do not travel through the outer core provides clear evidence that this layer is liquid.31