Which statement best describes P-waves and S-waves propagation?

• A. S-waves are shear waves that travel fastest
• B. S-waves are compressional and travel through solids and liquids
• C. P-waves are shear waves that travel slower than S-waves
• D. P-waves are compressional, fastest; travel through solids and liquids

Answer: (D)

The key idea is how P-waves and S-waves move and what media they can travel through. P-waves are compressional (longitudinal) waves, meaning the particle motion is in the same direction as the wave travels, like pushing and pulling along the path of travel. Because this motion is the fastest way for a disturbance to move through matter, P-waves are the quickest seismic waves. They can propagate through solids, liquids, and gases, so they move through both solids and liquids in Earth. S-waves, on the other hand, are shear (transverse) waves; the particle motion is perpendicular to the direction of travel, and they rely on shear rigidity to propagate. Liquids and gases don’t provide that rigidity, so S-waves cannot travel through them, though they do move through solids and are slower than P-waves. The statement that best captures these properties is that P-waves are compressional, fastest; travel through solids and liquids. This also explains why P-waves arrive first at a seismic station, followed by the slower S-waves.