How do P-waves and S-waves differ in propagation, and what do their speeds reveal about Earth's interior?

• A. P-waves travel only in solids
• B. S-waves are shear and cannot travel through liquids
• C. P-waves are compressional and faster; S-waves are shear and cannot travel through liquids
• D. P-waves slower than S-waves

Answer: (C)

Seismic waves reveal Earth's interior by how they move and how fast they travel. P-waves are compressional, meaning the particle motion is in the same direction as the wave travels; this allows them to propagate through solids, liquids, and gases, and they are the fastest seismic waves, arriving first at distant stations. S-waves are shear, with particle motion perpendicular to the direction of travel, and they require a material with shear rigidity. Because liquids can’t support shear stress, S-waves cannot propagate through a liquid layer like the Earth's outer core, so they disappear there and create a seismic shadow zone. The different speeds and the presence or absence of certain waves as they traverse Earth's layers let scientists infer layer boundaries and states of matter inside the planet. That’s why the statement that P-waves are compressional and faster; S-waves are shear and cannot travel through liquids best captures what these waves reveal.