Here's why:

* P waves (Primary waves) are compressional waves, meaning they travel by squeezing and stretching the material they pass through. They can travel through solids, liquids, and gases.

* S waves (Secondary waves) are shear waves, meaning they travel by causing particles to move perpendicular to the direction of wave travel. They can only travel through solids.

Because of their different mechanisms, P waves are faster. This difference in speed is a key factor in seismology:

* Locating earthquakes: Seismometers detect the arrival of P waves first, followed by S waves. The time difference between the two arrivals allows seismologists to calculate the distance to the earthquake's epicenter.

* Understanding Earth's structure: The way P and S waves travel through different layers of Earth helps scientists understand the composition and physical properties of those layers.