In seismology, what distinguishes P-waves from S-waves and what do their properties reveal about Earth's interior?

• A. S-waves travel only through solids and do not propagate through liquids.
• B. P-waves travel through all materials equally.
• C. P-waves are fast compressional waves that travel through solids and liquids; S-waves travel only through solids and do not propagate through liquids; their behavior reveals Earth's layered structure.
• D. S-waves can travel through liquids.

Answer: (C)

Seismic waves reveal Earth’s interior by how they move through different materials. The P-waves are compressional waves that push and pull particles as they travel, so they move by changing the volume of the material. They can propagate through both solids and liquids, and they do so the fastest, so they often arrive first at a seismic station. The S-waves, by contrast, shear particles side to side and require a solid to transmit that motion; liquids cannot support this shear, so S-waves do not propagate through liquid layers.

This difference is what makes their behavior so informative. Where a liquid layer exists, S-waves disappear or are greatly altered, creating shadow zones, while P-waves continue and refract at boundary layers, changing speed and direction. The combined observations of P- and S-waves thus map out Earth’s layered structure—crust, mantle, outer liquid core, and inner solid core—by showing where each type can travel and how their paths bend.