P-waves (primary waves) are compressional waves that travel through solids, liquids, and gases. They cause particles to move back and forth in the same direction as the wave is traveling.

S-waves (secondary waves) are shear waves that travel only through solids. They cause particles to move perpendicular to the direction of wave propagation.

Reasons for the difference in velocity:

* Type of motion: P-waves involve compression and expansion, which are more efficient in transmitting energy. S-waves, on the other hand, involve shearing motion, which is less efficient.

* Rigidity of the medium: P-waves can travel through both solids and liquids, while S-waves can only travel through solids. Solids are more rigid than liquids, allowing P-waves to propagate faster.

Therefore, P-waves travel at a higher velocity than S-waves.

Typical velocities:

* P-wave velocity: Around 6 km/s in the Earth's crust.

* S-wave velocity: Around 3.5 km/s in the Earth's crust.

This difference in velocity is why P-waves arrive first at a seismograph station after an earthquake, followed by S-waves. The time difference between the arrival of P-waves and S-waves can be used to determine the distance to the earthquake epicenter.