Which statement about seismic waves and Earth's interior is correct?

• A. S-waves propagate through both solids and liquids.
• B. P-waves propagate only through solids.
• C. S-waves cannot propagate through liquids, P-waves refract at internal boundaries, revealing a layered structure including a liquid outer core.
• D. Both P-waves and S-waves propagate identically through all materials.

Answer: (C)

Seismic waves reveal what Earth's insides are made of by how they move through different materials. S-waves are shear waves that require a solid to carry shear stress; liquids cannot support shear, so S-waves do not propagate through the liquid outer core. This absence on the far side of the planet creates an S-wave shadow zone that tells us there’s a liquid layer there. P-waves are compressional waves and can travel through both solids and liquids, though their speed changes with material properties. When they encounter boundaries between layers with different densities and states, like the core and mantle, their paths bend in a process called refraction. This bending and the change in travel times of P-waves map out a layered structure inside Earth, including a liquid outer core.

So the statement that fits the evidence is that S-waves cannot propagate through liquids, and P-waves refract at internal boundaries, revealing a layered structure including a liquid outer core. The other ideas don’t align with what we observe: S-waves don’t travel through liquids, P-waves aren’t limited to solids, and waves do not propagate identically through all materials.